Characters: The Binding of Isaac

A list of characters who appear in The Binding of Isaac. Currently under construction, please contribute!

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Playable Characters

    In General 

Isaac/Player Character

A little boy who gets thrown in the basement and must fight for his life against a never-ending assortment of demons and mutants, the alternative being his mother killing him. Beating the game in certain ways or collecting certain items unlocks variations of Isaac with their own unique attributes.

Story-related Tropes:

  • Break the Cutie: The entire premise of the game is one for him.
  • Children Forced To Kill: One boss is his own mother trying to murder him. He must do this in self-defense.
  • Create Your Own Villain: Well, antagonist. Isaac would never have been forced to kill his own mother if she hadn't snapped and tried to sacrifice him in the first place. Gains some very interesting implications at the true ending of Rebirth where it's revealed that Isaac becomes a demon or the Anti-Christ inside the Chest. Did Isaac have darkness inside him all along or did he decide Then Let Me Be Evil after the horror that he went through?
  • Dark and Troubled Past: The horrible memories of things that Isaac suffered through are relived in level loading screens. In one ending, the photograph montage of Isaac's past leaves a lot of interpretation as to what happened to his family, namely what happened to his father (who likely left, went missing, or died).
  • Dead All Along: After beating It Lives for the tenth time and unlocking Ending 10, Isaac comes across his corpse stuffed into a chest. At least one interpretation of this involves Isaac having been dead the whole time. The next three endings explore the events leading up to his death in reverse chronological order, first showing him willingly locking himself in the chest, then showing his self-loathing which led to this, and finally showing his tragic backstory through a heartbreaking Photo Montage.
  • Driven to Suicide: Isaac hangs himself upon finding Transcendence (a noose) in Ending 2. This is Played for Drama in the final few endings: After we see that Isaac has possibly been Dead All Along in the tenth ending, we are shown him willingly entering the chest himself due to his self-loathing over his Dark and Troubled Past.
  • Enemy Without: The boss of The Cathedral Isaac seemingly fighting himself and the boss of The Chest is ???, which was strongly implied to be a dead Isaac post-suffocation and cyanotic.
  • It's All My Fault: The ending you get for beating the Chest seems to imply that the cause of Isaac's self-loathing is because he blames himself for his sister Maggy's death. His parents, or at least his mother, seem to blame him as well.
  • Then Let Me Be Evil: It is strongly implied that at the ending of Rebirth, Isaac becomes a demon or the anti-christ in order to deal with all the horror he went through, up to and including his mother.

Gameplay-related tropes:

  • Acrofatic: Several items make your character fatter, and most don't negatively impact speed. Combine these with items that increase your speed and you have this trope.
  • Badass: Badass enough to kill Satan, in both his normal & most powerful forms.
  • Badass Adorable: He's still a child in the end of the day.
  • Blessed with Suck: Guppy's Tail makes you finds golden treasure chests more often. This comes at the expense of keys needed to open those chests, though if you're lucky the chests cough them up. Works wonders with an Skeleton Key or a Flat Penny.
    • Made potentially worse or better with The Left Hand item that changes all chests to Red Chests. They don't require a key to open, but half or more of the things that come out of a red chest is not something you want (including enemies and troll bombs).
  • Body Horror: Most of the powerups mutilate the character in some horrific way.
  • Bullet Hell: It's possible to get a character that shoots a four-way bullet spread at the speed of a machine gun with additional single shots from the other three cardinal directions.
  • Collision Damage: Isaac takes damage from pretty much everything that moves. The player can cause this if they use My Little Unicorn (or the Chariot tarot card) or the Virus powerup. The former is temporary, while the latter is permanent but doesn't prevent damage in return.
  • Dark Is Not Evil: An Isaac with a lot of Devil items is still just looking to survive.
  • Deal with the Devil: The player can trade heart containers to Satan for demonic items.
  • Detachment Combat:
    • When using The Shears or Transcendence (the latter is permanent), Isaac floats around as a severed head. The former also turns his body into a secondary attacker, but only works for the current room.
    • The Peeper pops out one of your eyes, which bounces around the room, hurting everything it touches.
  • Did You Just Punch Out Cthulhu?: Isaac can tear apart any number of bosses over the course of the game, including Satan.
  • Empowered Badass Normal: No matter which version you choose, you'll inevitably turn him into a supernatural being in the process of beating the game.
  • Eye Scream: Some items cause this.
  • Fantasy Character Classes: The characters are based on archetypal RPG classes (with the exception of ???)
    • The Hero/Balanced: Isaac.
    • The Cleric: Maggie.
    • The Druid: Eve.
    • The Thief: Cain.
    • The Mage: Judas.
    • The Berserker: Samson.
  • Faustian Rebellion: The items Satan sells you at the cost of your maximum health often end up making beating him a lot easier.
  • Flash of Pain: His sprite blinks when he takes damage.
  • Flight: Plenty of items, both permanent and temporary. You can become an angel, a demon, an angelic ghost, a demonic ghost, a floating head, and a cat (which somehow flies). All of these let you pass through rocks and over pits.
    • Becoming possesed by Guppy (the flying cat form) alows you to fly because why couldn't a possibly demonic cat possesion also allow you to fly. Also it's a kitty cat.
  • Full-Frontal Assault: Before the events of the game, his clothes were taken away from him as part of Mom's attempts to remove the evil around him. In most playthroughs, he fights his way through monsters naked.
  • Good Wings, Evil Wings: Both are possible.
  • Hammerspace: It's not specified where all the coins, keys and bombs are stored.
  • He Who Fights Monsters: As he fights through deformed monsters and demons, he will end up getting a lot of things that make him just like those deformed monsters and demons.
  • Ironic Name: "Isaac" means "he laughed" or "he who laughs." Not much laughing occurs. Quite the opposite, really. invoked
    • Although it was based off the biblical story. See Meaningful Name below.
  • Item Get: Holds up new items - even in battle. You're not invincible when that happens, either.
  • Kindhearted Cat Lover:
    • Isaac seems to adore Guppy the cat the most out of his (deceased) pets.
    • He seemed to also like Max (Cricket in Rebirth) second most out of the pets, or possibly equally to Guppy, because Max's head makes him cry hardest of all items.
    • There is also Tammy, who although she doesn't evoke as strong a response as as guppy, will still makes Isaac cry more than normal, translating into a 360 shot.
  • Meaningful Name: The name of all of the playable character relate to bible stories, with Isaac's name relating to the Binding of Isaac story the game is based on.
  • No Name Given: His surname is never known.
  • Paper-Thin Disguise: It is strongly implied in-game (and confirmed by Word of God) that all the characters are just Isaac putting on a wig, eyepatch, hat, etc.
  • Secret Character:
    • Collecting three Guppy items (Dead Cat, Guppy's Tail, Guppy's Paw or Guppy's Head) will transform the player character into Isaac's pet cat. Doing so unlocks a new item (the first time), gives you flight, and applies the effect of the Mulligan.
    • In Rebirth, the same is possible with "Lord of the Flies", activated by finding three fly-related items.
  • Swiss Army Tears: As a weapon!
  • Throw Down the Bomblet: The bombs he can carry around are pretty good for killing things as well.
    • Cartoon Bomb: Said bombs are round, have a fuse, and are marked with a skull.
  • Took a Level in Badass: Throughout the game, if the right items are acquired. Isaac starts the game as a crying child and can potentially end it as a flying demon that can demolish bosses in seconds with giant lasers.
  • Troubled Fetal Position: In the floor transitions and at the start of each floor.
  • Urine Trouble:
    • The aptly named Number One upgrade replaces his tears for a urine attack instead.
    • Also Lemon Mishap, which leaves a puddle that damages enemies.
    • Entering a room with half a heart makes the player leave yellow urine stains on the floor (unless playing as Eve, whose stains are red instead).
  • Winged Humanoid: Possible with certain items.

    Isaac 

Isaac

Debut: The Binding of Isaac
Named after the biblical son of Abraham, is playable from the start and has no defining accessories. According to Edmund McMillan, he represents the fighter class of traditional games and begins with no items.
  • Badass Normal: He's the only playable character that starts off without any sort of supernatural assistance. Even the D6 is merely a six-sided die.
  • Bald of Awesome: Being a young child, he has no hair normally. It doesn't hinder him at all.
  • Heart Is an Awesome Power: His unlockable starting item is merely a six-sided die. Many players agree that it is among the best starting items in the game regardless.
  • He Who Fights Monsters: Isaac himself is an alternate True Final Boss that can be fought instead of Satan. More details below.
  • Jack of All Stats: Average in all stats except for having 3 starting hearts.
  • Luck Manipulation Mechanic: The all-mighty D6 dice! An activated item capable of transmuting any collection related item(s) into other random item(s).
  • Master of None: Gets no starting item to work with, unlike the others. Unless you unlocked the D6.

    Magdalene 

Magdalene

Debut: The Binding of Isaac
Named after a friend of Mary, the mother of Jesus, she is distinguished from Isaac by blonde hair and bow. Maggy represents the cleric class from traditional games and starts with the Yum Heart: an activated item that restores one heart upon use.
  • Boring but Practical: Her gimmick is somewhat bland compared to the others, but having 4 red hearts to work with allows for Devil Room deals early on without severe consequences. Additionally, the Yum Heart restores a red heart every 3-4 rooms or so, which keeps her healthy and can be a godsend during the later chapters.
  • Combat Medic: Called "the cleric" in the art booklet.
  • Dead Person Impersonation: The death of the real Maggy seems to be what caused Isaac's family to fall apart.
  • Everyone Calls Him Barkeep: Usually referred to as Maggy.
  • Stone Wall: Starts with low speed and 4 hearts, the highest starting amount possible, and a healing item.
    • Taken Up to Eleven in Rebirth during "The Tank" challenge. Maggy now starts with seven heart containers, the Yum Heart, and the Infamy mask, which blocks (some) damage from the front. Unfortunately, she now runs slower than even some enemies, making dodging difficult, if not impossible.
  • Tertiary Sexual Characteristics: Longish hair and a bow.

    Cain 

Cain

Debut: The Binding of Isaac
Named after the biblical son of Eve and first murderer, he is distinguished from Isaac by an eyepatch. He represents the Thief class from traditional Role-Playing Games and starts the game with the Luck Foot: a passive item that improves random events in various ways.
  • Bald of Awesome: Just like standard Isaac.
  • Born Lucky: Cain's starting item gives him increased luck in certain situations.note 
  • The Dev Team Thinks of Everything: For most characters, Technology 2 stacks the beam on top of their tears. Cain's the exception, his tears being shut off due to his eyepatch. Chemical Peel is changed, too; while with other characters every other tear will have increased damage due to one half of the face being scarred, with Cain, every tear has a 50% chance of dealing increased damage.
  • Eyepatch of Power: He has an eyepatch, and also starts with higher attack that other characters.
  • Fragile Speedster/Glass Cannon: Starts with great speed and attack and an average rate of fire but suffers from low range and health. Outright called "the thief" by the art booklet. Being "the thief" could be considered Fridge Brilliance considering he "stole" his own brother's life in the Biblical story of Cain and Abel.
  • Nerf: His speed and damage got hit with the nerfbat for Rebirth, and the mechanics of Lucky Foot changing also indirectly hurt him.

    Eve 

Eve

Debut: The Binding of Isaac
Named after the first woman and original sinner, Eve is distinguished from Isaac by long dark hair and eyeshadow. Representing the druid class from traditional games, she starts with The Whore of Babylon, which turns her into a demon at low health, and the Dead Bird, which fights for her when she is damaged.

    Judas 

Judas

Debut: The Binding of Isaac
Named after the disciple who betrayed Jesus, he has a fez to distinguish himself from Isaac. He represents the mage class from RPGs and starts the game with the Book of Belial, an activated item that increases attack power for one room. In the original game, he unlocks after beating the Womb for the first time. In Rebirth, he unlocks after beating Chapter 5 for the first time.
  • Bald of Awesome: Just like Isaac.
  • Glass Cannon: More so then Cain. Starts with average rate of fire, range, speed and high attack power, further bolstered by his Book of Belial. In exchange, he starts with only 1 heart.
  • Deal with the Devil: While holding the Book of Belial, the chances of a Devil Room appearing are increased. Unfortunately, Judas' starting 1 heart makes it impossible to make deals unless he gets more heart containers or soul hearts.
  • Difficult but Awesome: Starts off with the most consistent damage of all the playable characters (Eve and Samson's powers are conditional), but with only one starting heart and average speed for dodging, you don't have much leeway to screw up.
  • Nice Hat / Take That: Edmund based Judas' Fez on Phil Fish, due to him voting against Edmund and his comrades at the Independent Games Festival, although Edmund later stated he did it cause it was funny.
  • Squishy Wizard: The artbook refers to him as the "mage", fitting in the other RPG archetypes. It fits since Judas has high starting damage (especially with the Book of Belial active) but very low starting HP.

    Samson 

Samson

Debut: Wrath of the Lamb
Introduced in Wrath of the Lamb and named after the biblical strongman, he has long, brown hair and Big Ol' Eyebrows. He also starts with a red headband called Blood Lust. This is fitting with the fact that he represents the Berserker class. An extremely important part of this character is how he was substantially changed. In the original Samson started with one heart container and gained a damage boost for every enemy he kills in a room. In Rebirth, he starts with three heart containers and gains a damage boost every time he takes damage.
  • Blood Knight: His trademark power-up is called Blood Lust, after all!
  • Conservation of Ninjutsu: Invoked by his special hair-band. The more enemies you face in a room, the more powerful you'll get! But single target rooms won't trigger any attack-boost. This is changed in Rebirth.
  • Critical Status Buff: In Rebirth, Bloody Lust makes Samson deal more damage as takes damage, with him reaching near maximum damage for having taken several hits in a row. In this case he literally Turns Red to reflect Bloody Lust activating.
  • Difficult but Awesome: Blood Lust makes Samson powerful... if you manage to survive the hordes of enemies that give him his strength. In Rebirth, this mechanic is changed.
  • Glass Cannon: With just 1 starting heart container, he's as fragile as (and under some circumstances, even more than) Judas. However, his special gear makes him extremely effective against large groups of enemies and/or mook makers. This is changed in Rebirth.
  • Martial Arts Headband: It's the source of his Blood Lust passive ability.
  • Power-Up Letdown: He's probably the most difficult character to unlock in the original (with ??? possibly being more tedious than difficult), as it requires you to pass on 2 free items on a regular run... and its capacity is dubious to say the least. After months of complaints, Edmund promised a buff to Samson. The buff was starting with a soul heart on top of his single regular heart. He became much more useful in Rebirth, as he now starts with three regular hearts and his buffs from killing enemies and taking damage remain until you leave the floor. On the other hand, now you have to clear two floors in a single run without taking any damage to unlock him, which is arguably even more difficult.
  • Took a Level in Badass: In Rebirth, Samson starts with a much more forgiving 3 hearts and gets to keep his buffs until he leaves that floor. This is a major improvement from his previous incarnation.

    Spoiler Character (BOI) 

???

Debut: The Binding of Isaac
Click here for the picture 

Unlocked after defeating Mom's Heart/It Lives ten times, ??? is a shout-out to Edmund McMillen's Newgrounds account (specifically his "Dead Baby Dressup" game). Starts with The Poop, which creates a pile of dung once per room. Whilst his official name is ???, his nickname is "Blue Baby", which is how he signs his death note on the Game Over screen (and how he's listed on the PSN Trophies list for Rebirth).

Despite being a Joke Character gameplay-wise, he is possibly one of the most plot-relevant characters in the game. His reveal indicates that the vignette of Isaac suffocating in the chest is canon, and not only that, but he somehow died of asphyxiation whilst there. At this point it is implied that Isaac has been Dead All Along and Endings 11 and 12 seem to indicate that Isaac actually locked himself in the chest after being Driven to Suicide.

    Lazarus 

Lazarus

Debut: Rebirth
Named after the one that was brought back from the dead. Unlocked in Rebirth by having four or more soul hearts at once. His gameplay is unique in that to gain a fairly powerful item and stat boosts, the game encourages you to kill yourself.
  • Back from the Dead: Starts play with one extra life.
  • Bloody Murder: After he uses his extra life, he gets the Anemic item, which lets him leave a damaging trail of blood for the duration of the room upon taking damage.
  • Crutch Character: One of the first characters available to unlock, he has a higher tolerance for damage than Isaac with the only apparent downside being his negative luck. After unlocking the D6, however, his advantage seems rather insignificant. Without using his extra life very tactically, he ultimately plays like a less lucky Cain or a less powerful Judas.
  • Jack of All Stats: Apart from his resurrection ability, he essentially plays like Isaac without a D6.
  • Meaningful Name: Who else but Lazarus would start with the ability to rise from the dead?
  • Not Quite Dead: His gimmick is being able to resurrect instantly, rather than outside the room he died (forcing him to restart it).
  • Power-Up Letdown: His extra life only resurrects you with exactly one heart container, and only gives you a blood trail for the current room if you get hit once. On the other hand, it does resurrect you on the spot (unlike the other extra lives that return you to the previous room), possibly allowing you to finish off whatever killed him.
    • Lazarus also receives a considerable boost in Luck, Range, Damage, and Speed, but still, that one heart container makes it a tricky run.
  • Red-Headed Hero: He has red-orange hair.

    Azazel 

Azazel

Debut: Rebirth
A demonic looking character whose name is associated with evil. Unlocked in Rebirth by making three deals with the devil in one run.
  • Action Bomb: He starts with only black hearts, which trigger the Death effect when depleted.
  • Bad Black Barf: His starting attack is a limited-range Brimstone.
  • Big Black Devil
  • Close Range Combatant: The closest thing that Rebirth has to one, since his starting weaponry forces him to get relatively close to enemies. He compensates by possessing above-average speed and flight.
  • Disc One Nuke: His flight and Brimstone are a major advantage in the early game, and flight remains an important bonus throughout.
  • Dungeon Bypass: Possessing flight lets him ignore spikes, rocks, and other terrain right from the start. Rather useful.
  • Fragile Speedster/Glass Cannon: Fast, possess flight, and has high starting damage, but at the cost of poor range that cannot be increased and having to charge Brimstone for attacks. His black hearts damage everything if one gets depleted and he can eventually gain red heart containers through Health Up items, but he is in overall much frailer than characters who can pick up red hearts from the get-go.
  • Hit-and-Run Tactics: Unlike others who depend on kiting with tears, Azazel often has to fly in, blast several enemies with brimstone, then get out of there before the rest retaliate.
  • Horned Humanoid: One of his horns is broken.
  • The Last of These Is Not Like the Others: Azazel is by far the most different from the other playable characters, since he starts with miniature brimstone instead of tears, has evil hearts (but can gain red ones, unlike ???), and has flight (which only The Lost shares). The result is a rather different playstyle, including item selection, since his smaller brimstone makes many items useless/redundant but others extremely powerful.
  • Necessary Drawback: Unless you pick up an item which outright overrides his Brimstone (which includes the actual Brimstone), his range can never increase, though it can be decreased. In addition, his short range negates any homing effects.
  • Required Secondary Powers: A melee character would've been an interesting addition to the roster, but an actual melee attack would make multiple rooms Unwinnable by Design. Azazel being equipped with the piercing brimstone and flight fixes both of these problems.
  • Winged Humanoid: Starts with flight.

    Eden 

Eden

Debut: Rebirth
Named after the biblical garden, Eden's stats are randomly generated every time you play as them. Their gender is undefined, and to play as them requires the use of one "Eden token", earned by beating Mom's Heart. Unlocks in Rebirth after beating the Womb for the first time.

    Lilith 

Lilith

Debut: Afterbirth
Named after Adam's first wife who defected from Heaven with Samael, nothing is known about her yet.

    Spoiler Character (Rebirth) 

The Lost

Click here for the picture 
Debut: Rebirth
A ghost character, and one of the only characters not based off of any sort of religious contextnote , next to ???. It is the secret character of Rebirth, and has a very complicated unlock process.

  • Chekhov's Gunman: Actually, it appears several times prior to being unlockable, albeit so briefly that few players would actually notice. Specifically, it is the ghost that departs from Issac every time he dies.
  • Glass Cannon: And how! It has zero health and can't gain any type of hearts, period. The only upside to this is that it take Devil Room and Black Market upgrades for free, possibly resulting in a One-Hit-Point Wonder powerhouse early on.
  • Guide Dang It: Unlocking it is a bit...convoluted. First, you need to let yourself get killed by a Mulliboom in the basement as Isaac. Next, you need to blow yourself up with one of your own bombs as Maggie in the caves, then let yourself get killed by Mom's foot as Judas. Finally, you need to let yourself be stomped to death by Satan's hoof as Azazel. And you need to do all of this in a row, without winning or dying in any other way. Mess up once and you have to start all over again. Not very easy, to say the least, although the at the very least you get (cryptic) hints as to the next step once you've completed one, and you can use seeds to make it easier, in contrast to almost every other unlock.
    • The "guide" was at least hidden in-game, though it would have been hard for one person to piece together. Dying in a sacrifice room while holding the Missing Poster trinket shows a puzzle piece as a cause of death. Putting the puzzle pieces together show the four game over screens required to unlock The Lost.
  • Joke Character: Starts with the ability to fly, with the downside of having absolutely no HP.
  • Lethal Joke Character: Its lack of HP means that it can take Devil items (some of the most powerful items in the game) for no cost at all. Normally it would cost one or two heart containers for one of those items.
  • One-Hit-Point Wonder: Dies in one hit from anything. Earlier versions had him die from continuing a run.
  • Power Up Letdown: Remember how some thought that Samson and ??? weren't worth the effort it took to unlock them? The Lost is quite possibly even worse in this regard, especially since unlocking it involves a dose of Guide Dang It too.
  • Walking Spoiler: Naturally, being the secret character of Rebirth.

Enemies and Bosses

Universal

    Enemy Types Introduced in the Original 
(Note: One asterisk means the enemy debuted in Wrath of the Lamb. Two asterisks means the enemy debuted in Rebirth, but isn't part of a new enemy type introduced in that game.)

Flies

Flies are among the most common enemy type throughout the game.
  • Airborne Mook: Making them immune to floor hazards, and allowing them to pass over obstacles.
  • Big Creepy-Crawlies: Even the weakest variety of fly (that you can actually fight) is still the size of one of Isaac's eyes.
  • The Goomba: Though these ones are killed by shooting at them with tears instead of jumping on their heads.

Black Fly/Attack Fly/Moter/Ring Fly**

  • Asteroids Monster: Moters split into two Attack Flies when killed.
  • Bulletproof Human Shield: As the Halo Of Flies and Pretty Fly powerups. The Forever Alone and Distant Admiration powerups spawn one capable of dealing good damage.
  • Mascot Mook: The black variety is featured prominently in the artwork, and easily has the most related items.
  • Mook Promotion: If allowed to approach a pile of poop, a black fly consumes it and turns into a Pooter.

Pooter/Fat Fly*

  • High-Pressure Blood: One of the first enemies you'll see that can fire blood shots.
  • Mook: Not any more dangerous than the normal flies.

Boom Fly/Red Boom Fly

  • Action Bomb: Black variation.
  • Disaster Dominoes: An efficient way to clear a Boom Fly room is to kill one of them while they're all flying in a cluster.

Sucker/Spit*

  • Action Bomb: The green variety.
  • Disaster Dominoes: In a room with several Spits, killing one while standing in the right direction will often cause a chain reaction of vomit bombs that kill the rest of them.
  • Taking You with Me: They release hard to dodge ranged attacks for Suckers or vomit bomb for Spits when killed.

Eternal Fly*

  • Invincible Minor Minion: Until the monster or poop they orbit is destroyed, at least.
  • Meaningful Name: As mentioned above the Eternal Fly is, well, eternal, until the poop or monster they orbit is destroyed.

Humanoid

Pacer/Gusher/Splasher**

  • Elite Mooks: Gushers to Pacers.
  • High-Pressure Blood: Gushers shoot out a fountain of blood from their necks, but only the occasional blood shot can actually deal damage.
  • Overdrawn at the Blood Bank: Gushers, as the name implies, have a constant stream of blood flowing from their necks.

Gaper/Frowning Gaper/Gurgle*/Skinny**/Rotty**/Null**

  • Creepy Child: Gapers; They look much like your player character, but with Tears of Blood and a creepy smile.
  • Elite Mooks: Frowning Gapers to Gapers and Rotties to Skinnies.
  • Eye Scream: Gapers have empty eye sockets filled with blood.
  • Tears of Blood: Gapers have them on their faces. Horfs have them and fire them to attack.

Horf/Psychic Horf**

Hopper/Fire Hopper/Leaper

  • Bullet Hell: Leapers shoot bullets in all four cardinal directions when they jump. If five end up in a room, be prepared to dodge.
  • Elite Mooks: Fire Hoppers and Leapers, the former being stronger and the latter shooting bullets. It should be noted that fire hoppers cannot spawn by themselves, Hoppers have to touch fire first.
  • Infernal Retaliation: Hoppers move twice as fast, jump twice as far, and inflict more damage when they're on fire.
  • Violation of Common Sense: Not only does every other enemy (or character, even) take damage themselves from fire, but Isaac's main combat strategy is attacking enemies with water. Despite that they seem to have double health.

Maw/Red Maw/Psychic Maw*

  • Elite Mooks: Red Maws, with their lips stitched, can't shoot but chase you pretty fast. They also get knocked away farther when hit, making it difficult to line up multiple shots against them.
    • Psychic Maws also have their eyes stitched. They have a Third Eye which shoots homing tears, and a Eternal Fly circling them.
    • Maws themselves are basically Horfs, only with the ability to move, which eliminates the tactic of staying just outside of their range and shooting.

Mr. Maw/Swinger**

Mulligan/Mulligoon/Mulliboom*/Hive/Swarmer*

  • Action Bomb:
    • Normal Mulligans puff up and explode into flies if they survive long enough and get cornered.
    • Mulligoons replace the flies with a 4-way shot plus a troll bomb.
    • Mullibooms come with bombs implanted in them and will charge and blow up on you.
    • The Swarmer spawns a Bomb Fly upon death.
  • Body Horror: The tumor-headed Mulligan grows more decayed as the flies inside it transform it into the Hive. From there, the Hive becomes a Swarmer, a rotten head filled with flies.
  • Incurable Cough of Death: Has a very distinctive-sounding one when it spits up flies to attack you.
  • Mook Maker: Hives occasionally cough up flies and spit them at you, and Swarmers constantly spawn small flies as well as Pooters and Fat Flies when damaged.
  • Taking You with Me: Unlike his cousins, the Mulliboom (the one with a bomb stuck in his head) will rush straight at you and try to suicide-bomb you.

Globin/Gazing Globin

  • Artificial Brilliance: They will move coordinately to trap you. Especially problematic in rooms with obstacles.
  • Elite Mooks: The versions with one eye are faster and have more health. Their meat pile also slides away diagonally a short distance to avoid being hit.
  • Pulling Themselves Together: When killed, they will become a pile of goo and then recover. Luckily, they will take less to kill next time.
  • Punny Name: with "goblin" and "hemoglobin".
  • Why Won't You Die?: Unless you finish off the pile of goo, this will happen. The main problem is attacking the goo while several others are chasing you.

Knights/Selfless Knights/Floating Knight**

  • Airborne Mook: Floating Knight are flying Knight heads, ignoring pits and barriers.
  • Attack Its Weak Point: The front of them is solid bone or petrified flesh. The only way to actually damage it is to hit it in the back of its head, the exposed brain.
  • Elite Mooks: Its second variety chases the player instead of wandering aimlessly. They have a crying Isaac face instead of a brain on their back.
  • Shield-Bearing Mook: As customary, the solution is to flank it where its shield can't protect it.

Vis/Double Vis*/Chubber*

  • Body Horror: They look like headless humans, only with a gaping hole on their chests that shoots blood beams.
  • Bloody Murder: Fires what appears to be blood in a wide beam across the room it's in.
  • The Computer Is a Cheating Bastard: A variation of them fires Lil' Chubby instead of a beam. Their Lil' Chubby can go through rocks, and deal damage on the return trip, but yours can't do either.
  • Elite Mooks: As of the Wrath of the Lamb DLC, Vis has a second form which shoots a beam in both directions.
  • High-Pressure Blood: They shoot huge streams of blood that pass through all obstacles. In fact, they're probably the first enemy you'll see that can use blood beams.

Baby/Angelic Baby*

  • Body Horror: Flying, blood-covered fetuses.
  • Elite Mooks: Has an angelic version that shoots a three-way spread shot instead of one bullet.
  • Fetus Terrible: Think of the familiar babies you can get, only shrieking and bloody.
  • Teleport Spam: Will frequently disappear into the nether before reappearing elsewhere a moment later.

Dople/Evil Twin

  • Doppelgänger: The Evil Knockoff variety.
  • Elite Mooks: The Evil Twin. Instead of looking like a meat puppet, it looks like a demon that can: fly, shoot a three spread, and do more damage. Good things is that its bullets travel slower, giving you time to dodge and shoot.
  • Enemy Without: They mirror the player's movements, including shooting, but most rooms come with a mechanism with which to destroy them. There are also bombs, and praying that whipped shots will work.
  • Foreshadowing: The appearance of these foreshadows the nature of Isaac the boss.

Fred

Lump

Grubs

Very large white maggot-like creatures. They actually seem to be miniature (or perhaps juvenile) versions of Chub.

Charger

  • Deadly Lunge: If they spot Isaac, they will open their mouths wide and speed towards him... exactly like Chub does.

Maggot

  • The Goomba: Has no attacks, and just wanders around the room randomly.

Spitty/Conjoined Spitty**

  • Multiple Head Case: The conjoined version has a head on both ends, letting it spit blood in both directions simultaneously.

Spiders

Added with Wrath of the Lamb. A faster alternative to the flies, though they're land-bound.
  • Big Creepy-Crawlies: Like with the flies, the smallest fight-able spider is still anot much smaller than Issac.

Spider*/B. Spider*

  • Asteroids Monster: Big Spiders split into two after death.
  • Demonic Spiders: Literally. Gameplay-wise, they're slightly harder to kill than files thanks to their erratic movements and greater speed. invoked

Trite*/Ragling**

  • Shout-Out: A Trite is a head on four legs, with an eye-like mouth in the center. In other words, a tektite.

Other

Clot/Clotty/I. Blob*

  • All Your Powers Combined: The I. Blob's projectiles, which are a combination of the other two.
  • Blob Monster: Meat blob, tar blob, and... whatever the I. Blob is made of.
  • Bullet Hell: They tend to shoot haphazardly, and very often, in every direction.
  • The Cameo: Clot is very similar to Gish.
  • Elite Mooks: The I. Blob variant shoots projectiles in all eight directions.

Host/Red Host

  • Attack Its Weak Point: Only in the grey variation. To hit them, you must wait for them to expose the flesh under the skull. Moving them over spikes will kill them.
  • The Cameo: From a multiplayer player flash game with the same name.
  • No Sell: Regular Hosts are protected by their skull when they aren't firing. Trying to hit them does nothing. Trying to bomb them just causes them to make a troll face.
  • Shield-Bearing Mook: The skulls the normal variety cover themselves with are impervious to pretty much any normal damage source.

Brain/MemBrain/Guts/Mama Guts*

  • Asteroids Monster: MemBrain and Mama Guts splits into two Brains and Guts respectively when killed.
  • Body Horror: Does a sliding brain or rolling ball of organs sound pleasant to you?
  • Organ Autonomy: Quite literally, they're organs that move on their own.

Poky/Slide*/Wall Hugger**

Stone Grimace/Vomit Grimace*/Gaping Maw**/Turret Grimace**/Stone Eye**/Brimstone Grimace**

  • Disaster Dominoes: Vomit Grimaces are often found together with Boom Flies, which are not immune to the vomit projectiles.
  • High-Pressure Blood: Brimstone Grimaces canshoot blood in a strong beam.
  • Invincible Minor Minion: The usually shut off after all other enemies in a room are dead, however.
  • Living Statue: Statue heads that fire projectiles.
  • Vacuum Mouth: Gaping Maws are constantly breathing in, which will suck Isaac and other things towards it.

Boil/Gut*/Sack*/Walking Boil*/Walking Gut*/Walking Sack*

  • Elite Mooks: As of Wrath of the Lamb, there is the Gut, which shoots green bombs, much like Sloth. As well as mobile versions of each.
  • Healing Factor: The non-mobile versions slowly regenerate over time when damaged. However, they spawn with less-than-full HP and can't attack until they've reached their maximum health. Their health is directly proportional to their size.
  • Mook Maker: Sacks are made of spider silk, which spits out spiders. It, too, has a mobile version.

Para-Bite/Cod Worm**

Eye/Bloodshot Eye

  • Elite Mooks: A second version which looks like a white eyeball and nothing else. It shoots blood beams instead of electrical beams.
  • Eye Scream: Being giant eyes, it's inevitable.
  • Faceless Eye: They're giant eyes that are found just lying on the ground.
  • High-Pressure Blood: Bloodshot Eyes fire thick blood beams.

Leech/Kamikaze Leech/Holy Leech*

  • Elite Mooks: A second version that has an eyeball and explodes when killed.
  • Orbiting Particle Shield: The Holy variant has an Eternal Fly guarding it.
  • Palette Swap: There's an alternate version of that that's angelic white. Not just tinted that way, either, but a separate enemy.

B. Licker*

Mask + Heart*

  • Attack Its Weak Point: The Mask is completely immune to all damage. The Heart isn't.
  • Organ Autonomy: The hearts are not defenseless: they will attempt to run away from Isaac and will defend themselves with a four-way blood burst if he gets too close.

Keeper*/Hanger*

  • Bandit Mook: When they hit you, you lose three cents. Only two of those cents fall onto the ground to be picked up again.
  • Orbiting Particle Shield: Each Hangar has an Eternal Fly defending it.
  • Shoplift and Die: They are based off of shopkeepers (and by extension, Greed), and they'll try to take your money.

    Enemy Types Introduced in Rebirth 

Level 2 Fly/Full Fly

  • Elite Mooks: As the name suggests, they are standard Flies, but larger and stronger.

Dip/Squirt/Dinga

  • Asteroids Monster: Dingas split into two Squirts when killed, while Squirts split into two Dips.
  • Toilet Humor: Regular poop lying everywhere not good enough for you? Now you can fight the poop!

Level 2 Spider/Ticking Spider

  • Action Bomb: Ticking Spiders explode when killed.
  • Elite Mooks: Like the Level 2 Fly family, they are improved versions of standard spiders.

Baby Long Legs/Small Baby Long Legs/Crazy Long Legs/Baby Crazy Long Legs

  • Mook Maker: Baby Long Legs make Spiders, while their Small counterparts create Sacks.

Wall Creep/Blind Creep/Rage Creep

Round Worm/Nightcrawler

Bony

Death's Head

  • Invincible Minor Mook: They keep bouncing around the room until every kill-able enemy in it is dead. They aren't immune to the Chaos Card, but why would you waste a Chaos Card on one?

One Tooth

  • Bat out of Hell: Doesn't look very menacing in comparison to most examples, but it attacks all the same

Lil' Haunt/Wizoob

  • Expy: With their attack method and name, Wizoobs are clearly based off of the original Wizrobes.

Fatty/Pale Fatty/Fat Sack/Blubber/Fat Sack

  • Acrofatic: While they're normally slow and bumbling, Fat Sacks have an attack where they jump high in the air and slam down.
  • Dead Weight: Fat Sacks are fat zombies.
  • Gasshole: Fatties can fart to push things away. Pale Fatties shoot blood when they do this.
  • Kevlard: They're decently durable, and that probably comes from their immense fat.

Mom's Hand

  • Evil Laugh: Mom's laugh is the cue that they're in the current room.
  • Mook Bouncer: In addition to dealing damage, they also pull you back the starting room of the floor if they grab you. This can actually be helpful for a quick trip back to the starting room on a dead end, if you have the hearts to spare.
  • Wall Master: They hide on the ceiling of rooms, eventually diving down on Isaac.

Tumor/Psy Tumor/Camillo Jr.

  • Shout-Out: Camillo Jrs. have their name based off of Jr. However, they look more like regular Kracko, so the Jr. is likely there to make the inspiration clearer.

Grub

Nerve Ending

Homunculus/Begotten

  • Blackout Basement: Begottens darked the room if they break free.
  • Breaking the Bonds: They become significantly more dangerous when they break their chains and can pursue you.

Oob/Black Maw

  • Shout-Out: They are directly taken from Super Meat Boy, another Edmund McMillen game. The former was an enemy, the latter was an obstacle.

    The Fallen 

The Fallen

  • Asteroids Monster: After about 75% of his health his gone, he splits into two versions of himself.
  • Bullfight Boss: Does this when he isn't shooting. The charge is stopped by shooting him enough times, after which he'll shoot blood in every direction.
  • Classic Video Game Screw You: It's uncommon, but this guy can be the boss of ANY dungeon save ones with predetermined bosses. He's notorious for showing up in the Basement/Cellar, often before you've had a chance to buff your speed and survive his charge.
  • Degraded Boss: In Sheol. He's later fought at full strength when the Satan fight begins.
  • Early Bird Boss: He can replace any boss that's not pre-set by the level. If you're unlucky he can appear on the first or second floor...
  • The Dragon: To Satan. He even acts as the first stage of his master's boss fight!
  • High-Pressure Blood: He shoots blood beams in all four cardinal directions at once for one of his attacks.

    Krampus 

Krampus

  • Bad Santa: Drops a Lump of Coal.
  • Chest Monster: Randomly appears in Devil Rooms. He's more likely to appear if you have already entered a Devil room, even more so if you've made a deal.
  • High-Pressure Blood: Just like the Fallen, but he'll do it if you pass by him in any direction rather than after a charge.
  • The Krampus: It's in the name, after all.
  • Palette Swap: Of The Fallen, except he doesn't split into two parts and is mildly weaker. He also has no charge attack, instead firing off his blood beams if you stick to any cardinal direction of him for too long. Killing him may drop the Head of Krampus, which lets you use this attack.

    Spoiler Character (Rebirth) 

Uriel and Gabriel

Secret bosses in Rebirth, a fight with one of twonote  Angels is initiated by bombing the statue in an angel room at any point throughout the game. Beating one unlocks a piece of a key each time, and beating both and combining the keys allows the player to unlock the true final boss of the game.

  • The Blank: Even when not in statue form, they lack faces.
  • Bolt of Divine Retribution: They shoot coherent beams of holy light.
  • Color-Coded for Your Convenience: Uriel wears silver or blue, Gabriel wears red.
  • Degraded Boss: Two Fallen Angels attack you as Mega Satan's flunkies. One interpretation of this is that they kept the pieces of the Golden Key safe. failed when Isaac took the pieces, and were subsequently punished for it.
  • Dismantled MacGuffin: They each hold a piece of the key required to open the Gold Gate.
  • Guide Dang It: Bomb the angel statue in an Angel Room to provoke it into a fight. Less aggravating than other examples since there are more in-game clues and it is often next to blue fires, which can be bombed for soul hearts, so the player might figure this out by mistake if nothing else.
  • Hidden in Plain Sight: A fight can be initiated at any point in the game, provided you have beaten either ??? or The Lamb.
  • The Last of These Is Not Like the Others: You can fight a different-colored Uriel after beating him and Gabriel, but it doesn't drop a key piece.
  • Palette Swap: Each one has different colored rags.
  • Personal Space Invader: Gabriel.
  • Winged Humanoid: Albeit without a face.

The Harbingers

    In General 
The four Horsemen of the Apocalypse, unlocked after beating Mom for the first time. Famine, Pestilence, War and Death each have the ability of replacing one of the bosses of the Basement, Caves, Depths and Womb respectively, while the Headless Horseman has the capability to replace any of these bosses, and Conquest was introduced as an alternate boss to Death.

    Famine 

Famine

    Pestilence 

Pestilence

  • Body Horror: No eyes, rotting body with swollen parts, filled with explosive, possibly-toxic fluids, and surrounded by maggots and flies.
  • Green Around the Gills: His entire body is sickly green, swollen, and leaking some sort of disease-ridden fluid. So is his horse.
  • Incurable Cough of Death: His horse coughs flies.
  • Losing Your Head: Only the body (still riding the horse) remains after the first part.
  • Mook Maker: Maggots at first, Flies later.

    War 

War

  • Bullfight Boss: After losing his horse.
  • Bull Seeing Red: He's red, and his horse has bull horns and charges.
  • Mad Bomber: One of his attacks drops bombs randomly.
  • Oh, Crap: After losing his horse, hit him enough times and he'll fall to the ground stunned, with a terrified look on his face.

    Death 

Death

  • Dual Boss: He hops off his ride on his second stage, which goes on ramming aimlessly while he hovers around spawning Knights.
  • The Grim Reaper: The skeletal kind, complete with black cloak and scythes.
  • Mook Maker: Creates Knights.
  • Shout-Out: One of his attacks is very similar to one that a certain other Death you may know of uses.
  • Sinister Scythe: He can summon flying, spinning scythes to attack.

    The Headless Horseman 

The Headless Horseman

  • All Your Powers Combined: The head has Famine's triple shot and War's triple charge, while the body is Pestilence minus spawning enemies. Plus, you fight two bosses at the same time, which is what happens when Death loses half his health.
  • Classic Video Game Screw You: He can replace any horseman. Are you on the Womb/Utero 1, with Book of Revelations and Level 3 Meat Boy or Bandage Girl, hoping to finally get the corresponding achievement for Level 4? Too bad, have a pony instead.
  • Degraded Boss: The head alone can appear in normal rooms once you reach the Depths.
  • Dual Boss: The boss is split into: his head, that charges and shoots small bullets, and his body, which shoots large bombs and leaves trails of blood.
  • Headless Horseman: Well, yeah.

    Conquest 

Conquest

The Seven Deadly Sins (Minibosses)

    In General 

The Seven Deadly Sins

  • Anthropomorphic Personification: Of their respective sins.
  • King Mook: The "Super" variants introduced in Wrath of the Lamb.
  • Inescapable Ambush: Unless you hold certain items, they won't show up on the map at all until you run into them, and you cannot leave unless you carry a teleportation item.
  • Quirky Miniboss Squad: They fight you individually, but in the any of the levels following the fourth chapter, you may encounter up to three different Sins to fight in the same room.

    Envy 

Envy/Super Envy

  • Asteroids Monster: When defeated, he splits into two heads, which each split into two smaller heads, which finally each split into two tiny heads. Super Envy splits into threes instead of twos, except for the smallest heads.
  • Damage-Sponge Boss: To make up for the fact that he is easily the least dangerous form, he has more aggregate health than the other Sins.

    Gluttony 

Gluttony/Super Gluttony

    Greed 

Greed/Super Greed

  • Bandit Mook: His shots (and mere contact with him!) take away a couple of pennies from you, while making you drop a variable amount on the floor that you can try to retrive... Just be careful while doing so, if you don't want to be killed in the process. Oh! And guess what? His mook-lookalikes have the same ability as him. Yay!
  • Breakout Character: He's the most memorable and ubiquitous of the The Seven Deadly Sins mini-bosses, and Wrath of the Lamb cemented his status by adding a quite a few new versions of him.
  • Chest Monster: Replaces shops and secret rooms. He's more likely to appear as you go further in the game.
  • Expy: He shares some traits with Spelunky's Shopkeeper, including his multi-shot ability, potential to turn hostile on the player without previous warning, and overall dickishness.
  • Flunky Boss: Normal Greed summons Hoppers, Super Greed summons Keepers (which are Greed heads).
  • King Mook: Wrath of the Lamb added Super Greed, who is bigger, meaner, scarier, and can spawn Greed-heads. Said Greed-heads also appear in the Depths, the Catacombs, and the Necropolis.
  • Mook Promotion: Inverted. The game's standard version features him as a mini-boss, but the expansion added mook-versions of him that share some of his most aggravating attributes.note 
  • The Scrappy: Invoked. Given how he can replace shops and secret rooms, can knock out your money when he hits you, and can be fought multiple times in one run, players usually don't enjoy finding him.
    • He becomes an Ensemble Darkhorse if you have the D20.
    • You'd be hard-pressed to find anything in the game that will make you madder than fighting Greed in the Depths/Necropolis 2 or later and having him drop the Steam Sale.
  • Shop Keeper: When he's not out for your blood anyway.
  • Wingding Eyes: Super Greed has cent signs for eyes.

    Lust 

Lust/Super Lust

  • Body Horror: Super Lust's horribly deformed face.
  • Bullfight Boss: She does nothing but charge at you. Thankfully, she can't turn on a dime and there are plenty of rocks around to kill her momentum.
  • The Smurfette Principle: The only one of the Sins/Super Sins that has been identified as female.

    Pride 

Pride/Super Pride

    Sloth 

Sloth/Super Sloth

    Wrath 

Wrath/Super Wrath

  • Cartoon Bomb: His bombs are identical to yours, skull marking and all.
  • Expy: Of Bomberman.
  • Hoist by His Own Petard: Wrath's bombs can be pushed around like player-dropped bombs, allowing you to push them back towards him for a nice chunk of damage. This doesn't work on Super Wrath's troll bombs, though. Even more amusingly, any items that affect bombs will affect the bombs dropped by Wrath/Super Wrath. This can result in hilarious situations where, if you have the Bobby-Bomb collectible, every bomb the boss drops proceeds to walk back toward him.
  • Mad Bomber: Throws bomb after bomb at you. Since he's Wrath, this doubles as a Stealth Pun.

    Ultra Pride 

Ultra Pride

  • Creator Cameo: He's a stylized representation of artist Edmund McMillen, while his familiar is one for programmer Florian Himsl. Hence why he's named Ultra Pride.
  • Death by Cameo: You do fight them, after all.
  • Dual Boss: He comes with his familiar, and both have to be defeated.
  • Meaningful Name: The creators are ultra proud of their product. Additionally, putting yourself as a cameo in your own game is a flagrant show of pride.
  • Palette Swap: He's basically Sloth with different moves, while the familiar is a graphically altered Baby with a different shot type.

Chapter One: The Basement/The Cellar

    Monstro 

Monstro

Debut: The Binding of Isaac
  • Asteroids Monster: His Eternal form in the Eternal Edition splits as it takes damage. The smaller forms shoot less bullets, however.
  • Body Horror: He's some kind of deformed head, lacking a nose and most of his teeth.
  • Degraded Boss: After the game is beaten 6 times.
  • The Dev Team Thinks of Everything: One spacebar item, Monstro's Tooth, calls in the aforementioned boss to body-slam enemies. Guess which boss you can't use it against.
  • Dual Boss: The red version is two smaller Monstros.
  • Warm-Up Boss: One of the less complicated bosses and one of the most likely to run into first. Many prefer getting zero-damage boss battles on Monstro to increase likelihood of Devil Rooms spawning.

    The Duke of Flies 

The Duke of Flies

Debut: The Binding of Isaac
  • Body Horror: It's safe to assume that the only reason he can move around is because of the flies that have infested his bloated body. He also lacks eyes of any kind.
  • Degraded Boss: After unlocking "Everything's Terrible!"
  • Flunky Boss: "Attacks" by creating tons of flies.
  • Mook Maker: As its name implies, it creates flies.
  • Taking the Bullet: Spawned flies circle the Duke for a bit before coming after you, letting them soak up your shots.

    Gemini/Steven 

Gemini/Steven

Debut: The Binding of Isaac
  • Bullfight Boss: The big one can tire out after a few moments of running. If the big one is killed first, then the small one will turn red, give chase, and never tire out. Steven's little version will do that before the big one goes down.
  • The Cameo / Mythology Gag: Steven is from Time Fcuk.
  • Dual Boss: One big version that charges the player and one small version that shoots bullets.
  • Last Ditch Move: Both parts of Steven shoot bullets in all directions when they die.
  • Polar Opposite Twins: The big one is aggressive and chases you; the little one looks calm and makes ranged attacks until you kill the big one, at which point it charges at you.
  • Turns Red: What the small one does when the big one dies. Steven, on the other hand, has it do this at 1/3rd health.

    Larry Jr. 

Larry Jr.

Debut: The Binding of Isaac
  • Asteroids Monster: Not so much as other bosses, but if the middle section is killed first in a 5 body one, it will split into two more versions with just two segments.
    • Much easier to happen with his Eternal counterpart in the Eternal Edition, where a broken-off segment can last as just a head, and can regenerate segments.
  • The Cameo/Mythology Gag: A smaller version of Larrie's Lament.
  • Degraded Boss: The most common boss enemy to find in The Womb.
  • Dual Boss: Easily the most common one; his standard fight in the Basement is always two of him.
  • King Mook: As of Wrath of the Lamb, smaller enemies similar to it appear.
  • Warm-Up Boss: Shares this with Monstro, but which boss is the easier of the two if any is up to personal opinion.

    Widow 

Widow

Debut: Wrath of the Lamb
  • Body Horror: She's made out of a head with four eyes, leg bones, and toes.
  • Flunky Boss: Not only does she spawn spiders directly, but she can also spawn Mook Maker spider silk Boils! And that's not even getting into the ones that can spawn bigger Asteroids Monster spiders or other kinds of Boils.
  • Giant Spider: She's a larger version of the Trites. Considering that those are already the size of Isaac...
  • Glass Cannon: Relatively low HP for a boss, but moves unpredictably and spits out the equally unpredictable spiders, making her harder to beat without taking any damage than most bosses early on.
  • King Mook: To the spiders.
  • Rush Boss: She's a fragile boss, but if you don't kill her quickly, you'll get overwhelmed by the spiders she summons and the speed she moves at.

    Pin 

Pin

Debut: Wrath of the Lamb
  • Back from the Dead: Inverted: Pin was introduced in the Wrath of the Lamb as a weaker, living version of Scolex.
  • Tactical Suicide Boss: Is not immune to its green exploding projectiles, and due to a quirk in hit detection, can actually damage itself with them when it begins to bury underground. The dark grey one averts it, replacing the bombs with bullets.
  • Warm-Up Boss: Competing with Larry Jr. and Monstro for easiest boss in the game. Relatively low health, a large hitbox, extremely predictable and avoidable attacks, and he's not immune to his own projectiles. Many players feel silly if they take any damage to this boss at all, let alone die to him.

    Gurdy Jr. 

Gurdy Jr.

Debut: Wrath of the Lamb

    Dingle 

Dingle

Debut: Rebirth
  • Adorable Evil Minions: His squirts.
  • Bullfight Boss: One of his attacks is to charge at you three times in a row, occasionally leaving a pile of poop on the first charge. He scrunches up just before the charge.
  • Cute Is Evil: He's probably the cutest boss in the game, and not in an Ugly Cute way. But just like every other boss, he still wants you dead.
  • Degraded Boss: Often appears in the Womb.
  • Nobody Poops: Averted, obviously.
  • Not-So-Innocent Whistle/Enemy Summoner: He occasionally will whistle innocently (complete with a music note hovering above his head), before one to three Squirts appear beside him to fight. This is immediately followed by a bullet spread attack.

    The Haunt 

The Haunt

Debut: Rebirth
  • Flying Face: That rips itself open to reveal a skull when its minions are dealt with.
  • Flunky Boss: He is surrounded by three Lil Haunts which attack before he does. First one attacks, then the other two do. He's invulnerable until they're dead and he moves to the top of the room.
  • King Mook: To the Lil Haunts.
  • No Fair Cheating: He normally paces from side to side along the top edge of the screen, shooting downwards. If you try to move to the top where he can't shoot you, he'll immediately charge at you to cause damage.
  • Our Ghosts Are Different: This one has a skeleton. Which he'll gladly show to you.

    Gurglings 

Gurglings

Debut: Rebirth
  • Bullfight Boss: They'll charge you if you get in their line of sight.
  • Creepy Twins: When fought as a pair. Identical and dangerous.
  • Degraded Boss: They appear as regular enemies as early as the Caves, and don't even get a health bar. On the other hand, they constantly leave behind damaging blood trails (as opposed to only doing so when charging) and shoot six-way blood bursts rather than spawning flies, making them actually slightly more dangerous than the boss version.
  • Dual Boss: They appear as a pair.
  • Enfant Terrible: They seem to be even younger versions of Gurdy Jr.
  • Expy: The latest version of Gurdy, this time taking the form of two twins.

Chapter Two: The Caves/The Catacombs

    Fistula 

Fistula

Debut: The Binding of Isaac
  • Asteroids Monster: It has three distinct forms. The final part either spawns a Charger or a Boom Fly, depending on the version fought.
  • Body Horror: It's named for a real life chunk of Body Horror in which organs try to absorb each other.
  • Degraded Boss:
    • Appears as this in later stages. Most of the time, though, it will already be broken down to its second or third stage.
    • In Rebirth, the boss is now a Chapter One boss.

    Gurdy 

Gurdy

Debut: The Binding of Isaac
  • Body Horror: She's what can only be described as a mountain of flesh and organs, with a smiling face at the top.
  • Damage-Sponge Boss: The bulkiest boss in the Caves, having more health than some of the later bosses.
  • Degraded Boss: Interestingly, the weaker version attacks very differently, charging and firing circular waves of bullets. Contrast with the other bosses made into minibosses, who are more or less identical to their original version, but weaker. Wrath of the Lamb separates the degraded form into a different boss: Gurdy Jr. But The Very Definitely Final Dungeon DOES feature Gurdy as a normal Degraded Boss.
  • Flunky Boss: Spawns Flies and Boils to assist her.
  • Stationary Boss: She always stays in the middle of the room. Even if you push her away with a fear effect, she'll move back into position when it wears off.
  • Wake-Up Call Boss: When it comes to the pre-Wrath of the Lamb bosses who gave new players trouble, Gurdy is the most infamous.

    Peep 

Peep

Debut: The Binding of Isaac
  • Eye Scream: It detaches its eyeballs when its HP starts getting low. They block your shots and will inflict Collision Damage on you if you touch them... Oh! And they're indestructible until you kill their original owner.
  • Detachment Combat; His eyes will fly off as he takes damage, becoming wall-bouncing obstacles.
  • Extra Eyes: His Eternal counterpart in the Eternal Edition has six eyes. Yes, they can all detach, but they're also destructable.
  • Meaningful Name:
    • Pronounce the second p as though you were saying the letter "p".
    • There's also the fact that "peep" can mean to look at something, and he attacks you with his eyes.
  • Psychopathic Man Child: He smiles during the fight and cries after being defeated.
  • Toilet Humor: Attacks with urine.

    Chub/C.H.A.D. 

Chub/C.H.A.D

Debut: The Binding of Isaac

    Blighted Ovum 

Blighted Ovum

Debut: Wrath of the Lamb
  • Back from the Dead: Gemini is now a zombie, and is accompanied by his conjoined brother's ghost. The image appears to be more of a sperm and an egg. That wouldn't discredit this trope, it would just be a different set that came back, because at least the large one is undeniably dead no matter what perspective you look at it from.
  • Body Horror: The larger one has a gash on his head when his eye once was, lacks a lower jaw, and is either covered in blood or has no skin on his lower body.
  • Bullfight Boss: Like his living counterpart, except he never tires.
  • High-Pressure Blood: The ovum fires a blood beam if you stay in front of it for too long.

    The Hollow 

The Hollow

Debut: Wrath of the Lamb

    Carrion Queen 

Carrion Queen

Debut: Wrath of the Lamb
  • Attack Its Weak Point: Can only be hit from behind, except when exploiting its other weak point (see Feed It a Bomb below).
  • Back from the Dead: A mostly-skeletal Chub.
  • Dung Fu: Randomly produces Red Poop that damages if you touch it. Also can bounce off the poop as if a wall.
  • Feed It a Bomb: Same as Chub, though it's much more difficult once she starts moving diagonally.
  • Turns Red: Will bounce aimlessly around the room once her health is down to half.

    The Husk 

The Husk

  • Back from the Dead: The Duke of Flies' rotted corpse.
  • Flunky Boss: Spawns even MORE flies than the Duke does.
  • Rush Boss: Like his weaker counterpart, he has low health. Unlike his weaker counterpart, he can overwhelm you with flies if he has the chance.

    The Wretched 

The Wretched

Debut: Wrath of the Lamb

    Dark One 

Dark One

Debut: Rebirth
  • Blackout Basement: One of his attacks is to make the entire screen dark and hide himself in the shadows.
  • Bullfight Boss: He has a charge attack from one side of the screen to the other which angles at the player, similar to the charge attacks of Famine and War but harder to dodge.
  • Degraded Boss: In Sheol.
  • Frickin' Laser Beams: Will shoot Brimstone straight down if you stand below him.
  • Offscreen Teleportation: He can dash past the edges of the screen like the Harbingers.

    Mega Fatty 

Mega Fatty

Debut: Rebirth
  • Acrofatic: Normally slow and fat, but it has a leap attack which is executed in under a second.
  • Damage-Sponge Boss: Noticeably soaks up more punishment than most normal bosses.
  • King Mook: Of the regular Fatty enemy.
  • Selective Magnetism: One of his attacks is to magnetise everything in the room to him, mimicking the effect of the item Magneto.
  • Vomit Indiscretion Shot/Waterfall Puke: After the magnetism attack, he vomits up a barrage of projectiles that cover a portion of the screen.

    Mega Maw 

Mega Maw

Debut: Rebirth

    Polycephalus 

Polycephalus

Debut: Rebirth

Chapter Three: The Depths/The Necropolis

    Loki 

Loki

Debut: The Binding of Isaac
  • Hoist by His Own Petard: If you can kill his Boom Flies as soon as they spawn, they will blow up on him.
  • Flunky Boss: Summons Boom Flies periodically.
  • Tactical Suicide Boss: The Boom Flies it spawns will often explode on him, given how powerful you should be by the time you fight him.
  • Teleport Spam: Frequently teleports away during battle.

    Monstro II 

Monstro II

Debut: The Binding of Isaac

    Gish 

Gish

Debut: The Binding of Isaac

    Mask of Infamy 

Mask of Infamy

Debut: Wrath of the Lamb
  • Attack Its Weak Point: The mask is invincible until you destroy its heart. Even after that, you need to hit it from behind.
  • Interface Screw: The only boss without a health bar in the original version, though fixed in Rebirth. The fact that it's not easy to lay a shot in the last stage makes the fight look depressingly long.
  • King Mook: A beefed-up version of a standard enemy also added in Wrath of the Lamb.
  • Shout-Out: Inspired by Carmillia's mask in Castlevania.

    The Adversary 

The Adversary

Debut: Rebirth
  • Back from the Dead: He is the post-humous version of the Dark One, covered in bandages and with a shattered horn.
  • Blackout Basement: Can turn the screen dark for his attacks.
  • Dishing Out Dirt: He can jump up in the air and slam down, creating a circle of sharp rocks around him.
  • Frickin' Laser Beams: He can now shoot Brimstone in any cardinal direction, but don't think you're safe if you stand at a diagonal, because it curves towards you.

    The Gate 

The Gate

Debut: Rebirth
  • Back from the Dead: He is a post-mortem version of Mega Maw, as the flesh from his face leaks to the lower half of his head, his skull remains animated for the time it's alive.
  • Flunky Boss: Spawners a series of Leapers as a lead-up to the next trope.
  • Frickin' Laser Beams: One of his attacks is to spawn three Leapers and then fire a Brimstone laser that connects between the three and deals a whole heart of damage.
  • Meat Puppet: When defeated, something that looks like a tiny Isaac peeks outside a crack right before expiring.

    The Cage 

The Cage

Debut: Rebirth
  • Back from the Dead: He is a post-mortem version of Mega Fatty, with his head being absent (assuming it got chopped off) and his body rotting, along with a new stitch across his stomach, making him resemble a Mega Vis.
  • Dishing Out Dirt: His jumping attack now creates sharp rocks like the Adversary's, but he creates lines of them that travel outwards in several directions until they hit a wall.
  • Mega Mook: He's basically been turned into a Vis, making him this.
  • Mook Maker: He can spawn discoloured Vis that fire sickly gray Brimstone lasers.
  • Rolling Attack: He can curl up in a ball and roll around the floor in an attempt to hit Isaac.

    Mom 

Mom

Debut: The Binding of Isaac
Isaac's mother. In a fit of rage/insanity/fanaticism/whatever you think it is, she attempted to use a knife on Isaac, forcing him to flee to the basement.

Story-related tropes:

  • Abusive Parent: Takes away all her son's stuff, locks him in his room, and then tries to kill him. Convinced by God to do so or not, that's all the evidence you need.
  • The Bad Guy Wins: It's revealed that Isaac suffocated to death while inside the chest. She wanted Isaac to die because of an (alleged) order from God, do the math. However, subsequent endings revealed that Isaac locked himself in the Chest before the events of the game, and Rebirth's Ending 15 shows what appears to be her grieving for Isaac, so this is probably a subversion.
  • Big Bad: Ultimately subverted.
  • The Extremist Was Right: She's a religious fanatic that appears to think that she's on a mission from God, which is clearly not the case. At least until the canonical end of Rebirth, strongly implying that Isaac either was or now is a demon. If you subscribe to the "just batshit insane" theory, then she's Right for the Wrong Reasons.
  • The Fundamentalist: She's heaily Christian, and her hearing the voice of God is what kicks off the plot.
  • Hidden Depths: Ending 13 reveals that she used to be completely sane, and the caring and loving mother of a fully-functional family. However, as the family fell apart, so did her sanity.
  • Knife Nut: She's holding her knife in almost every cutscene she appears in, and you can unlock said knife as a usable weapon in-game.
  • No Name Given: Unlike Isaac, even her first name is unknown.
  • Offing the Offspring: Her ultimate goal.
  • One Bad Mother: Bonus points for actually being the main character's mother.

Gameplay-related tropes:

  • Attack of the 50-Foot Whatever: Her boss fight.
  • Ax-Crazy: Her goal is to kill her own son, and she doesn't sound too upset about it during the fight, what with the laughing and all.
  • Body Horror: The way Mom's body parts move throughout the fight (such as both eyes appearing on opposite sides of the room while she tries to stomp on you) suggest an ability to contort herself in rather...impossible ways.
  • Boss Banter: ISAAAAAC!
  • The Dev Team Thinks of Everything/Gameplay and Story Integration: Using the Bible instantly defeats her in her boss fight.
  • Death by Irony: Crushed by her own bible!
  • Disc One Final Boss: Demoted to this on your second playthrough and beyond.
  • Evil Is Visceral: Likely the reason her body parts are detailed and somewhat deformed.
  • Evil Laugh: She lets out one every time she successfully damages Isaac.
  • The Faceless: Her entire face is never shown at any point in the main game (the intro and epilogue don't count, as they're Isaac's cartoon drawings). This is subverted in Wrath of the Lamb, but only near the very end, and only briefly. In photographs, to be precise.
  • Fan Disservice: When Isaac uses the Mom's Bra item, a picture of his mother's shirtless torso flashes on the screen. A very unflattering picture of his mother's less-than-fit shirtless torso.
  • Fat Bastard: As shown by her giant leg and the picture that shows up when using Mom's Bra.
  • Final Boss: In your first playthrough. She's nowhere near the True Final Boss, though.
  • Flunky Boss: She'll summon groups of random enemies to battle Isaac
  • Giant Foot of Stomping: Her main method of direct attack.
  • Gonk: See that leg up above? That's all the proof you need.
  • Gross-Up Close-Up: One of her legin the boss portrait, and one of her body in the picture that appears when using Mom's Bra.
  • Humanoid Abomination: Most of what she does in-game and in a few cutscenes blatantly disregards normal body movements if not physics. She can step on her own eye, generally alter size, and her arm can fit in a chest inside her heart inside her womb.
  • Leitmotif: Thine Wrath... in the original, Matricide in Rebirth.
  • Mook Maker
  • Mother of a Thousand Young: If you go by the theory that the majority of the enemies are her former children.
  • Say My Name: ISAAC!
  • Tactical Suicide Boss: It's not actually mandatory, but making her stomp herself is possible, and deals lots of damage.
  • Womb Level/Bigger on the Inside: After defeating her, you will unlock her womb as a playable level.
  • Your Size May Vary: Just how big is she? In the cutscenes, she's a normal-sized human being, but her underwear is the right size to be worn by a child visibly smaller than her, in the boss fight her leg is the length of the screen, and then her womb is big enough to have two entire levels in it, with her heart being larger than Isaac.

Chapter Four: The Womb/The Utero

    Scolex 

Scolex

Debut: The Binding of Isaac
  • Attack Its Weak Point: You can only damage it by hitting it in the very back segment.
  • Back from the Dead: A posthumous version of Pin. He is what Pin would become had he been digested.
  • Fragile Speedster: It has the lowest health of any boss in the game, but it makes up for this with its speed and hard to hit weak point.

    Blastocyst 

Blastocyst

Debut: The Binding of Isaac
  • Asteroids Monster: As it gets damaged, it splits into smaller blobs. The smallest blob variety leaves behind the worm-like creature inside when destroyed.
  • Blob Monster: Judging by the name, it's a gigantic, blob-like cell.
  • Bullet Hell: Can happen if too many of them are left onscreen. Since they only fire in a set direction, however, it's pretty tame compared to most other examples in the game.
  • Degraded Boss: Very odd type. Blastocysts can be found in earlier levels as a Zero-Effort Boss. Later on, though, they can be found in their full forms.

    The Bloat 

The Bloat

Debut: Wrath of the Lamb
  • Back from the Dead: What remains of Peep's bones are broken, and his eyes are swollen.
  • Detachment Combat: Like Peep, but from the beginning.
  • Eye Scream: His eyes are covered in red growths, and aren't attached to him.
  • High-Pressure Blood: He'll fire two giant beams of blood from his eye sockets if you so much as put a toe in front of or beside him.

    Teratoma 

Teratoma

Debut: Wrath of the Lamb
  • Asteroids Monster: Except now filled with spiders instead of Chargers.
  • Back from the Dead: The Fistula has shriveled up and now shelters spiders.
  • Body Horror: Like Fistula, it's named after a real life Body Horror phenomenon in which the body forms a growth containing all three germ layers... one of which can potentially manifest as bone shards and teeth!

    Lokii 

Lokii

Debut: Wrath of the Lamb
  • The Artifact: With the "Double Trouble" mechanic in Rebirth, Lokii is kind of redundant; the fact that the halves summon Red Boom Flies is the only way they're different from a Double Trouble Loki fight.
  • Back from the Dead: Loki has been split in two, but both halves live on.
  • Dual Boss: He's the equivalent of fighting two Lokies at once, only with Red Boom Flies.
  • Flunky Boss: Unlike standard Loki, they summon Red Boom Flies. This gives them an advantage, because they won't be blasted in the face if their minions die near them.
  • Half the Man He Used to Be: They're Loki after being split in half, and somehow they're completely fine.
  • Teleport Spam: Just like normal Loki.
  • Visual Pun: The last two "i"s of his name have a line crossed through them on the bottom, making them resemble part of the Roman numeral II.

    Daddy Long Legs/Triachnid 

Daddy Long Legs/Triachnid

Debut: Wrath of the Lamb
  • The Cameo/Mythology Gag: Triachnid is a reference to a previous flash game by Edmund McMillen. Furthermore, Triachnid is an alternate of Daddy Long Legs; in his own game, Triachnid was finding a home for his young.
  • Dummied Out: As hinted here, Triachnid was originally was cut before the game was released. It wasn't removed entirely, though. Due to a glitch, it originally would replace the Cathedral's True Final Boss. This was eventually corrected, and Triachnid was added in as a normal boss.
  • Giant Spider: Bigger than any other spider in the game, taking up more that the screen can shown.
  • King Mook: A giant spider, same as Widow.
  • The Dev Team Thinks of Everything: There is an item that summons the Daddy Long Legs to stomp enemies in an area the player is standing in without hurting them. Using this against the Daddy Long Legs reskins it as the Triachnid's claw. Using it against Triachnid flat out doesn't work.

    Mama Gurdy 

Mama Gurdy

Debut: Rebirth
  • Ascended Meme: Apparantly a reference to a fan-made animation.
  • Expy: The fourth form of Gurdy to appear.
  • Spikes of Doom: She is able to create spikes around the room.
  • Wall Master: She can hide in the walls of the boss room, immune from damage.

    Mr. Fred 

Mr. Fred

Debut: Rebirth
  • Flunky Boss: Starts the fight accompanied by four Homunculi whose bonds he can break at will, and spawns Freds throughout the fight.
  • King Mook: A giant boss version of the enemy Fred that appears in Chapter Four.
  • Stationary Boss: Never moves from the centre of the room.

    Spoiler Character (BOI) 

Mom's Heart/It Lives

Click here for the picture The final boss of the fourth chapter, and the initial final boss of the game overall, after traversing the Womb, Isaac is forced to fight his mother's still-beating heart. Beating it is what unlocks Endings 1-10 (or 1-11 in Rebirth). After reaching Ending 9 (11 in Rebirth), Mom's Heart is replaced by a new boss: It Lives, Isaac's fetus.
  • Boss Banter/Say My Name: Has the same voice lines as Mom.
  • Boss Rush: It Lives has the ability to spawn Degraded Bosses such as Monstro, Larry Jr. and Chub to fight you alongside the enemies.
  • Bullet Hell: Rebirth's version shoots red tears nearly constantly throughout the whole fight, even when retreating to the back of the room.
  • Damage-Sponge Boss: Its lack of mobility is made up for by a big health bar. Its Eternal version in the Eternal Edition have even more.
  • The Dev Team Thinks of Everything: Using the Bible instantly kills either form like it did with Mom.
  • Enemy Without/Fetus Terrible: It Lives, which has been confirmed by Word of God to be Isaac's embryo.
  • Flunky Boss: It Lives can also make Degraded Bosses.
  • Get Back Here Boss: Despite not moving, it accomplishes this by occasionally retreating above the screen and leaving you to deal with its flunkies for a while.
  • Leitmotif: ...Be Done in the original, Ventricide in Rebirth.
  • Mook Maker: Hides at the back of the room while it spawns normal enemies to fight you, then jumps back to the centre to spawn more.
  • Nerf: Inverted; both bosses are made much harder in Rebirth.
  • Not Quite Dead: It seems that Mom survived their fight, or at least her heart did.
  • Stationary Boss: All it can do movement-wise is pull itself upwards to hide from attacks, making bombs very useful against it.
    • The Eternal form in the Eternal Edition averts this greatly; it will pursue you relentlessly around the room until it dies, still spawning enemies.
  • Tactical Suicide Boss: In Rebirth, it moves to the top of the screen instead of disappearing entirely. Anything that wanders up there, enemies included, takes damage.
  • True Final Boss: Was initially this to Mom's Final Boss. However, this is subverted by the Halloween update, and further subverted by Wrath of the Lamb and Rebirth, to the point that it's no longer anywhere near the real final boss.

Chapters Five and Six: Sheol and Dark Room

    Spoiler Character (BOI) 

Satan

Click here for the picture 
Debut: The Binding of Isaac
The original True Final Boss (although this isn't true as of Wrath of the Lamb and Rebirth), Satan himself is the boss of what was originally the game's secret floor. Despite offering Deals with the Devil throughout the game, he turns on Isaac, and tries to kill him. His boss fight is comprised of three phases, detailed below under Sequential Boss.

  • Attack of the 50-Foot Whatever: His final form is him flying far above the screen and stomping on Isaac with giant feet ala the Mom boss fight.
  • Black Speech: During the fight with him, he sounds somewhat like this. They are actually Mom's quotes, played backwards and in a much lower tone.
  • Deal with the Devil: He offers them throughout the game before you reach Sheol.
  • Evil Laugh: Since he uses Mom's voice clips, he can do this.
  • Flunky Boss: His first form is spawning the Fallen to fight you instead.
  • Giant Foot of Stomping: His third phase has him attempting to crush Isaac with his giant hooves. They work like Mom's but he does it much faster.
  • Leitmotif: Enmity of the Dark Lord in the original, Hericide in Rebirth.
  • Living Statue: His second form is a unique boss where the statue of Satan comes to life, grows much larger and attacks Isaac with tears and lasers.
  • The Man Behind the Man: Implied, after all, he did at the very least string Isaac along through the Deals he makes in the devil rooms earlier on, before turning on Isaac at the last minute.
  • No Sell/Press X to Die: Using the Bible on him, which killed the Mom and Mom's Heart instantly, kills you instantly instead. The same applies to later bosses too.
  • One-Winged Angel/Sequential Boss: Has three forms (see Flunky Boss, Living Statue and Attack of the 50-Foot Whatever respectively).
  • Satan: It's in his name, and he looks the part as well.
  • Say My Name: Satan's battle cry when you defeat his first phase is Mom's Boss Banter, only lower pitched and in reverse.
  • Sequential Boss: Three phases: first a fight with The Fallen, then a fight with Satan himself on the ground with you, and finally a fight against his giant stomping legs.
  • Tactical Suicide Boss: Those exploding enemies he spawns don't just hurt Isaac...
  • Theme Music Power-Up: When you fight the Fallen, his first phase, the regular boss theme plays. When Satan actually starts fighting, however, Hericide kicks in.
  • True Final Boss: He was the final boss of the game in the Halloween update, but this has been since trumped by Wrath of the Lamb and Rebirth.

    Spoiler Character (Rebirth) 

The Lamb

Debut: Rebirth
  • All Your Powers Combined / Confusion Fu: The Lamb volleys of shots mirror those of previous bosses. This combined with how it switches the type of bullet with each volley, and that all of it's bullets are the same color, make it a very difficult opponent to predict.
    • Shadow Archetype: The Lamb's attacks also resemble combinations that can be used by Isaac - for example, Monstro's Lung plus Homing Tears. It's Detachment Combat is a lot like the Pinking Shears. It even begins the battle in Isaac's crying position!
  • The Antichrist: In the Bible, the Lamb was a symbolic reference to Jesus. In this game however, the Lamb is an undead demonic creature fought after Satan. The implication is that this Lamb is a demonic parody of Jesus.
  • Creepy Cool Crosses: Specifically, an upside-down one on his forehead.
  • Dem Bones: The Lamb is essentially a demonic skeletal child with curved horns.
  • Detachment Combat: His second form involves his head floating around the room attacking you while his body lies limply on the ground spawning flies and sometimes tossing out Monstro-style shots.
  • Dual Boss: His second form.
  • Flash Step: One of his attacks qualifies as this.
  • Frickin' Laser Beams: His second form gains the ability to shoot four-way Brimstone in the cardinal directions. He can also start rotating the beams like Mom's Heart/It Lives.
  • Horned Humanoid Skull for a Head: He looks like an undead demon.
  • Leitmotif: The Fallen Angel
  • Losing Your Head: During the second phase of the battle.
  • Mythology Gag: His name is a reference to the expansion for the original Isaac, named Wrath of the Lamb. He is also visually similar to Wrath of the Lamb's horned skull logo. Given that Edmund had more planned for the original Isaac, it's possible he was always intended to be the last boss of the expansion, but flash's limitations meant he couldn't add him in.
  • Nightmare Face: It's skeletal face isn't pleasing to look at, especially in the vs. screen.
  • Sequential Boss: Has two forms.
  • True Final Boss: Averted. He was available from the launch of Rebirth, but so was Mega Satan, meaning he never qualified as the True Final Boss.

Alternate Chapters Five and Six: The Cathedral and The Chest

    Spoiler Character (Wrath of the Lamb) 

Isaac

Click here for the picture 
Debut: Wrath of the Lamb
After beating It Lives, the Cathedral is unlocked. At the center of the Cathedral resides Isaac himself... and one of the toughest challenges in the game.

    Spoiler Character (Wrath of the Lamb) 

???

Click here for the picture 
  • The Cameo/Mythology Gag: From Edmund McMillen's flash series Dead Babies.
  • Flunky Boss: One way he varies from the previous boss is that he summons flies in addition to his other attacks.
  • Leitmotif: In Rebirth, in order to further differentiate ??? from the preceding boss, he's given his own track, Ascension.
  • Invincible Minor Minion: In Rebirth he gets a halo of flies, which block your tears.
  • Palette Swap: He is very similar to the previous boss, Isaac, so most of the tropes above apply to him too.
  • Teleport Spam: Utilized in his third phase in Rebirth, with a tendency to reappear right on top of the player.
  • True Final Boss: Of Wrath of the Lamb, being the boss of the only sixth chapter in the game, and since beating him unlocked the game's real ending. Subverted in Rebirth.
  • Weaponized Teleportation: Tries to teleport into the player during his third phase in Rebirth.

The True Final Boss

    Spoiler Character (Rebirth) 

Mega Satan

Debut: Rebirth
  • Bragging Rights Reward: Besides unlocking the final and definitive ending, there is no reason to fight him other than to boast you did it. The only thing you get out of winning is a new picture for your save file. There are no associated item or character unlocks, not even a mark on the character file to indicate that character beat him.
  • Bullet Hell: What some of his attacks delve into.
  • Final Boss Preview: Regular Satan's third phase can be considered this. In Sheol, you only fight his lower half. In the Chest/Dark Room's secret room, you face the much more difficult upper half.
  • Flunky Boss: Summons The Harbingers, the Super Sins, and fallen versions of the Angels.
  • Guide Dang It: The only in-game hint you have to his existence is "The Angels await!." You're supposed to bomb the Angel statues in the Angel Rooms, prompting them to engage you; both will drop a piece of a Golden Key on defeat, and getting both pieces opens a Golden Door in the Chest/Dark Room that leads to Mega Satan.note 
    • Fortunately, completing the key also unlocks "Dad's Key" as a regular item, which can also be used on the Golden Door for future playthroughs.
  • Leitmotif: Hericide
  • Marathon Boss: Compared to the normal 30-60 second boss fight, he takes 4-5 minutes unless you have the Chaos Card, and that's if you have a good enough run.
  • Red Eyes, Take Warning: Considering he's the (current) True Final Boss of the game, you will want to take warning.
  • Sealed Evil in a Can: Heavily implied to be sealed by the beings in the Angel Rooms.
  • Sequential Boss: Including his summons, he has twelve phases.
  • Soundtrack Dissonance: Due to a bug, doing too much damage to any form will cause the calm, somber "boss defeated" music to start playing.
  • True Final Boss: The very definitive True Final Boss of the game (which is saying something, considering there have been seven other candidates before it), having an extra unlock process required each run.