The player character. Awakened by the minions to rebuild and re-instate a reign of terror after his predecessor was slain by heroes. Armed with a suit of heavy plate armor, a massive weapon, and an army of minions he now seeks revenge against the heroes who slew the previous overlord. He was actually one of the heroes himself, and was treated to a Disney Death when the tower started collapsing. His allies left him for dead and the minions recovered the amnesiac hero and converted him to evil.
Anti-Villain: A low corruption Overlord is a merciless force towards threats, and seems to very much enjoy the power and position he holds, but he never raises a violent hand towards innocents, and at several points makes choices that are decidedly heroic. Arguably, a 0% corruption Overlord may have memories of his life as a hero.
Dirty Old Man: Over the course of the games, Gnarl gets very vocal about his attraction to the mistresses - to the point it actually earns him a thump on the back of the head from both Rose and Kelda.
The Dragon: Gnarl is one of the few recurring characters in the whole series, and since minions can live for an unknown amount of time, he's had the chance to serve at least four different Overlords - possibly many more.
Pragmatic Villainy: His In-Universe justification for any good choices in the first game. Make yourself look good to fuel a long-term evil empire.
One of the Overlord's potential mistresses, Rose is an order-obsessed stick-in-the-mud who compliments a low-corruption Overlord. Mid-game you have the option to spurn her for her sister, Velvet. In Overlord II, Rose serves Emperor Solarius as the leader of the Sentinels, a fanatical group dedicated to hunting down magical creatures and magicians across the world; as it happens, Rose isn't following the anti-magical doctrine, and only works for Solarius because he represents the best chance the world has at lasting order and peace. She's also canonically the first Overlord's lover and mother of the second game's Overlord.
Big Damn Heroes: When the Wizard returns and tries to steal your magic and kill you, Rose (if you didn't spurn her for Velvet) interrupts him by attacking the tower heart, giving you a chance to get your magic back and go after him.
Face-Heel Turn / Heel-Face Turn: In an attempt to restore balance to the world, she joined Solarius in his campaign of world domination during the second game. However, when he shows signs of becoming too corrupt, Rose quickly changes sides and offers her aid to the Overlord.
In the Hood: As leader of the Sentinels, she wears a hooded cloak.
Scare 'Em Straight: In the second game, she tries to scare her son off the path of the Overlord by showing him the ruins of the Tower. It doesn't work. Eventually, she decides that he's Necessarily Evil.
Took a Level in Jerkass: She's suffered one by in the backstory to Overlord II, apparently induced by the disappearance of her lover and the destruction of the Tower. Not only does she effectively abandon her own son in Nordberg and leave him to be bullied for the first few years of his life, but she also performed a full-fledged Face-Heel Turn by helping Solarius in his campaign against magical creatures.
Woman Scorned: On the flipside... If you spurned her for Velvet, she betrays you in revenge and allows the Wizard into your tower.
Rose's sister. Velvet is a sadistic, money-hungry seductress whom the Overlord rescues from William the Black. After saving her, she offers the Overlord to take her as his mistress and give him the time of his life, at which point he can spurn Rose and take her instead.
Big Damn Heroes: When the wizard returns and tries to steal your magic and kill you, Velvet interrupts him by attacking the tower heart, giving you a chance to get your magic back and go after him.
What Happened to the Mouse?: After taunting the Overlord when he escapes from the collapsing Abyss portal, he disappears from continuity entirely, necessitating a new Jester in the second game. Chances are good he was killed offscreen for his betrayal.
Bob the Farmer
An easily-victimized peasant, Bob is one of the few humans to own territory outside the heavily-fortified village of Spree, and has suffered terribly because of it; halflings have invaded his farm and staked him out on the pumpkin patch as an improvised scarecrow. As the expansion pack reveals, his troubles don't stop there...
Butt Monkey: First, he's tied to a scarecrow frame and left to watch as his entire crop of pumpkins is destroyed. Then the Mellow Hills are invaded by Kahn. Then he's kidnapped by halflings all over again, this time being dragged into hell.
The Schizophrenia Conspiracy: Under the impression that pumpkins can speak and are working with the halflings. Nevermind that halflings actually eat pumpkins...
As of Lord Spree's death in the attack on Spree Castle, Archie the innkeeper has been made the de facto mayor of Spree Village; with the village under siege by halfling raiders and no supplies but alcohol, he naturally beseeches the Overlord for help in recovering the food that the halflings stole from them.
A religious brotherhood established in the city of Heaven's Peak, the priests of the Order have barricaded themselves inside their fortress-like temple to keep out both the zombie hordes infesting the city and the remaining civilians seeking refuge.
A God Is You: After conquering their defenses, the Silent Order immediately declare you their new god. It even ends up as one of your titles!
Barrier Warrior: During the attack on their temple, the priests bolster their defenses with magical force fields.
Black Comedy: People with lips sown shut should be terrifying, but the Silent Order is played for laughs.
Corrupt Church: Not actively villainous, but certainly self-obsessed, given that they're leaving dozens of potential worshipers to die outside their walls.
Large Ham: He could give Gnarl a run for his money.
The Seven Heroes
Seven legendary heroes who, many years ago, slew the previous Overlord. However, as the years passed they grew complacent in their fame and gradually succumbed to their selfish desires, becoming villains themselves in the process.
The Big Bad: The Wizard. He lead the Seven Heroes to kill the previous Overlord and is still the leader of the group.
The Dragon: Gnarl, and - depending on who the Overlord spurned - either Rose or Velvet
The Evil Genius: Goldo Golderson. The only one who relies on a war machine to fight in his boss battle.
The Brute: Kahn, Melvin. Kahn was the Warrior of the group back in the day, and is exactly what you'd expect from that type of adventurer. Melvin is just too big to be anything but muscle and has no desires beyond gorging everything in sight.
The Dark Chick: Jewel, Sir William. Jewel was the Thief in the original group. Sir William used to be a Paladin, but is now the head of a hedonistic cult.
Token Good Teammate: Oberon. Formerly the Archer, Oberon did not willingly fall from grace like the others. He came back from the adventure tired of fighting, and would not raise his bow to defend Everfree Forest from the Dwarves. He then got assimilated by a magical tree and caused the entire forest to be corrupted.
I'm a Humanitarian: According to captured peasants, Melvin's appetites have extended this far.
Pop goes the Halfling: When you kill Melvin he explodes into a disgusting mess of bile; when you find him in his Ironic hell, this become a gameplay element.
Ironic Hell: For his gluttony, he wound up in a hell of killer pumpkins and exploding sheep, where he eats endless amounts of food and is unable to stop until he literally explodes... only to be re-incarnated and made to repeat this over and over and over again for all eternity. At the end of his abyss, the Overlord leaves him in a pit, where it turns into "starve for eternity", which is still ironic.
Jabba Table Manners: Disgusts even the Minions, who urinate on everything and wear dead rats for hats.
Rolling Attack: Halfway through his fight he decides to just roll over the Overlord and his horde.
Trash of the Titans: His mansion is decidedly messy; numerous corridors and hallways have been converted into improvised storehouses for all the dirty dishes and table scraps that the feasts constantly produce.
Villainous Glutton: Melvin is so gluttonous that he's carved out an entire empire from the surrounding countryside just to feed his appetite.
An Elven ranger and one of the heroes. After the Overlord's fall, Oberon succumbed to Sloth and slept while the Dwarves raided his home and wiped out his people. As a last subconscious act, Oberon's body became a great tree, blocking passage to keep the Dwarves from further destroying his people's home.
And I Must Scream: Oberon's semi-conscious and knows what he's doing to the forest, but is unable to do anything about it.
Heavy Sleeper: He only wakes up after all of his tree's roots are killed.
Ironic Hell: His abyss; he's trapped in his tree form and forced to watch his subjects massacred over and over in a (badly acted) play about the genocide of the elves.
Mook Maker: Creates some kind of feral rat creatures from his nightmares. Get used to these guys while you travel in Everfree.
Token Good Teammate: The only one of the heroes who expresses regret over what they've become, and the only one not enjoying himself; furthermore, the Wizard later reveals that when the Eighth Hero fell to his apparent death, Oberon was the only hero who insisted that they try to rescue him. Even his sin (simply tiring of conflict) is rather light compared to the others.
Tragic Villain: The Elf may have caused most of his species to die by not wanting to fight, but it's considerably more sympathetic than the other heroes who laughed as they went down the slippery slope.
When Trees Attack: While Oberon doesn't want to fight you, the tree sure does. It's the driving force in the boss battle; beat the tree down so you can kill Oberon.
Woobie, Destroyer of Worlds: One Elf is tired of fighting and hangs up his bow. Entire species is nearly killed as a result. Gets controlled by a giant tree in the time while the Overlord is sleeping. As it's a magic tree, it turns the entire forest into a literally nightmarish place populated by Skulls and Elf ghosts. And Oberon cannot end it himself.
Your Mind Makes It Real: Besides all the elves in the forest being, well, dead, there's also the small matter of Oberon's nightmares taking on a physical forms as monsters called Skulls.
Sir William the Bold/Black
A human paladin, Sir William succumbed to the sin of Lust and makes all of Heaven's Peak suffer for it, leading a cult devoted specifically to sex. The succubi queen he summoned to please him, his cult, and anyone else interested is the source of the Heaven's Peak zombie plague. He's also Velvet's betrothed, though she really doesn't give a damn about him.
The Dwarven king who, predictably, fell to the sin of Greed; driven by hunger for gold, he attacked Evernight to loot the Elvish temple treasures, and kept only a handful of Elves alive- the males to work as slaves in his gold mines, the females to be his consorts. In combat, he uses a giant steampunk tank called Rolly.
Bling of War: Goldo totters into battle wearing a suit of golden armor and a golden crown, and wielding a golden axe for good measure. Both the crown and the axe become collectibles for your minions after his death.
Ironic Hell: For his Greed, Goldo gets turned into a living statue of solid gold. Though, the one calling the shots in the Abyss is fascinated when the Dwarves' fury at Goldo for getting them sent to the Abyss overcomes their greed, and is content to let Goldo get pummeled by his own once-loyal subjects.
Slave Race: He started a war between the Elves and the Dwarves because he wanted the Elves' gold. Goldo made the remaining Elven men into a workforce to mine more gold. And then he forced the remaining women to be his consorts.
Screw This, I'm Outta Here: Subverted; after seeing the Overlord and his minions approaching, Goldo beats a hasty retreat... only to return in the driver's seat of Rolly.
A human thief, Jewel suffers from the sin of Envy, and uses her skill as a thief to steal whatever she can. With an army of mercenaries and beholders led by Kahn at her back.
Classy Cat-Burglar: Leads a band of ninjas and just steals everything, nailed down or not.
Get Back Here Boss: You don't actually fight Jewel in a boss battle. You have to chase her down to her hideout, then herd Jewel onto the nearby Tower Portal to nab her.
Green-Eyed Monster: Got something you value? Even if it's just a old teddy bear, Jewel wants it. What you treasure, she wants out of nothing more then jealousy.
What Happened to the Mouse?: When you capture and interrogate her, she just... disappears from the game. (Gnarl mentions something about an Iron Maiden, which means her getting forgotten probably meant she died from it, oops?)
A human warrior, Kahn is possessed by Wrath and leads an army of beholders and mercenaries alongside Jewel.
Huge Guy, Tiny Girl: In their scenes together, he towers over Jewel by several feet. And then somehow gets bigger for your boss fight.
Ironic Hell: Less so than the others, but still fitting. When you find Khan in the final Abyss, he's been turned into a wall, unable to fight or express his anger in a physical form. He doesn't seem to be too miffed about his situation, oddly enough, maybe because he was finally able to sit still long enough to realize rage won't solve anything at that point.
Mighty Glacier: If you've seen berserkers in other games, they're normally fast. Kahn can't do much more than a slow walk, but that mace will take out groups of minions at a time.
A human wizard and the first to turn evil in succumbing to the sin of Pride, and subsequently the one to corrupt all the other heroes. He's actually the Overlord who was killed by the heroes, having performed a Grand Theft Me on the original Wizard and purged his soul from his body. He used the player to kill the heroes after corrupting them and leaving them vulnerable while he recovered. When Kahn is killed, he takes that as his cue and returns to the Tower to steal back his power and kill you.
Smug Snake: Pride's his hat. This had to come into play somewhere.
Thanatos Gambit: His death allowed him to possess the Wizard and get close to the heroes, corrupting them into the aggressive menaces encountered over the course of the game; once his replacement had done his dirty work in getting rid of them, he can take back his throne with no opposition from any of his former "allies."
A Wizard Did It: The Wizard being the corrupter of all the heroes was probably a play on this meme.