Ichigo's various hollow forms are often called "Ichithing".
Ichigo's bankai clothing gets nicknamed "The Pimpcoat".
Ichigo's inner hollow personality is called "Shirosaki"note Ichigo's surname is Kurosaki which means "black promontory" and "shirou" means "white" due to the inner holler being Ichigo in negative, "Hichigo"note Hollow Ichigo, "Whichigo"note White Ichigo, "Ogihci"note Ichigo spelled backwards, "our DLHF"note Dear Little Hollow Friend, which comes from the Chronicles of Narnia, and "Hollowichi".
The Chapter 350 hollow form is sometimes called "Vasto Lorde Ichigo", "Mullet Hollow", "Bull Ichigo", "Demon Ichigo", "Hell Knight Ichigo", "Super Hollow Ichigo"note Due to the length of his hair, like Goku when he goes SSJ3, "Luna Cortante"note Zangetsu's name in Spanish and "Ichigonator"note A play on "Terminator" due to the thrashing he gave Ulquiorra.
Tensa Zangetsu bore a remarkable resemblance to Byakuya so gained the nickname "Zanbyakuya". Often used derisively.
Due to a lack of sensical naming scheme being found, "George" became the nickname of the Tensa Zangetsu/Inner Hollow fusion.
The Chapter 420 form has gained nicknames such as "Hollow Cow", "Ichigetsu", "Getsugo" and "Ichigod".
Ichigo's constant power upgrades whenever he reaches the point of defeat or death has earned him the nickname "Holy Lord Ichigo".
One chapter's cover showing concept art for Ichigo's super-hollow form earned Ichigo the nickname "Scarmask".
Chapter 540 generated new nicknames for Ichigo's weapon: Bachgetsu, Juhagetsu or Zan!Bachnote Due to the chapter's discussion of Zangetsu's similarity to a younger Yhwach from a thousand years ago.
"Main Characters" is often a sarcastic nickname for Chad, Uryu, Orihime and Rukia due to the main cast being Out of Focus so often.
Arrowlad is the most common nickname due to Uryu being an archer.
White - when he teamed up with Renji against Szayelaporro, fans called them Red & White.
Poor Chad: Not the character, but the treatment of him, especially in the filler.
Dragon Boobs - due to her massive pair of breasts, and that her special power is akin to the most common wish used when using the Dragon Balls: reviving the dead.
Old Man Genocide, for his role in the destruction of both the Quincy and the Bount. It also becomes relevant when his bankai is revealed to preserve a memory of every single person his zanpakutou is ever killed, allow him to create a skeletal zombie army.
Also "Captain Triggerhappy".
Captain "TREASON!", for his completely illogical decisions for damning someone. He actually doesn't use the word often, but fans have exaggerated it.
As one of the oldest and most powerful characters in the story, he's often called Captain Grandpa.
Also "Soi Bean", "Soi Milk", "Soi Vay" and "Soi Phone" as a play on her name's English pronunciation.
Her bankai is often nicknamed "Banzooka" or the "Stinger Missile"note Due to it beinga combination of her hornet theme and possessing a giant missile launched from her arm, leaving her too weighed down for effective combat, but granting her a hell of an attack range.
Squad Three gets nicknamed "The traitor squad" Rogue Squadron, because three captains in a row (counting fillers) turned their backs on Soul Society for various reasons.
After her background is revealed, she obtains new nicknames: "Truepachi"note After being revealed as the first Kenpachi, "Sadakohana"Explanation : Based on her appearance in Chapter 523, she resembles the character Sadako from Ringu. and "Captain Annihilation"note She has a terrifying face and has mastered every sword style in existence.
IceFail, due to the ongoing joke that ice-attacks are often useless in anime and manga in combination with his "jobbing".
Captain Bankai Whore, since he never bothers with his Shikai, every time after his first fight he uses Bankai. This nickname is also used for Ichigo and Byakuya and any other character who frequently uses bankai in battle.
The Christmas Tree - One of his Bankai's special abilities that he uses to freeze Harribel. Guess what it looks like?
"Captain Kid", due to his young age. Bonus points for being one letter short of Captain Kidd.
Captain Grumpypants, due to his perpetually grumpy mood.
Godpachi was first used in response to Narita's God-Mode Sue version in the novel "Spirits are Forever with You". It became entrenched after the post-invasion Training from Hell he's put through by Unohana, to unlock his full strength.
Afro-san - Bonus points for this actually being started by the manga itself and catching on with the fans (Keigo uses it). Also Afrogami.
Hachibei, the bald man from the group, is called Captain Chang Koehan.
Aizen and cohorts
Rogue Captains - Aizen, Ichimaru, and Tousen.
Also referred to as The Traitor Triumvirate.
Or the "Bad-Eyesight-Trio". Because, you know, one has glasses (and destroys those dramatically), one never opens his eyes and one's blind...
Captain Broken, for having abilities too powerful for an anime character: for instance, stopping the main character's sword attack with one finger. Also known as "Captain Superman/Evil Superman", as a reference to his invincibility and to his Evil Makeover resembling Clark Kent's switch to Superman (minus the suit).
The Other Sasuke - Their first names sound alike and they both betray their people, not to mention being utterly massive Base Breakers.
Aizen's strand of hair that isn't pulled back is known as The Bitchcurl.
His evolved form has picked up a few less-than-respectful ones, such as "Condomman, the Walking Condom, Condom!Aizen, Tube Sock Ninja, Captain Broken Condom", and "Grand Dragon" for his resemblance to a Ku Klux Klan member.
And now that that form was revealed to be a sort of cocoon that he was in while he completed his transformation, his newer form has picked up several new names, including "Butterflaizen" and "Captain Mullet"
As of chapter 415, "Butterflaizen" seems even more appropriate for his newer newer form, as he has grown 6 Narmy butterfly wings.
And now, as of chapter 419, in a new Hollow-like form with questionable openings all over his body - Ladies and gentlemen, I give you: Vagizen. Less NSFW-ish(and as a result, less common), we have Abominaizen
And to get to his Mullet form and Butterflizen form, Aizen is said to have gone through "Gagafication".
The Other Hazama - Named for the now famous Expy of himself who is currently the God of Trolling.* Both characters are voiced by Doug Erholtz in their respective English dubs, albeit Doug only took over temporarily for Erik Davies.
"HAG" (Hot Arrancar Girl) - Before Tier Harribel's name was revealed, she was known to many as this.
Before she transformed the nickname was Underboob.
"Aunt Harribel", as her name was originally translated as "Tia Harribel". "Tía" means "Aunt" in Spanish.
Or Halle Berry.
Her three Fraccion (Emilou Apacci, Cyan Sung-Sun and Franceska Mila Rose) have recently garnered the name "Tres Bestias," in-universe epithet given by the Hollows to refer to how they served as Harribel's enforcers. Fans have since followed suit in calling them by that title.
Before that happened, though (and after it happened too), these three would be called The Halibabes (from "Halibel", an incorrect romanization of their mistress' name).
The Saddest Clown in the World/Mexico* See below nickname for Hueco Mundo- Ulquiorra Cifer, a stoic Arrancar whose Facial Markings, Perpetual Frown and chalk-white skin make him look like a crying, whiteface clown.
Ulq[Insert Random gibberish here] - because some people don't like trying to spell his name.
Ulqiblahblah - For The Stoic, he sure does talk a lot.
"Emo Clown" is another popular one.
Also, "Emocar" (his facial markings bear a resemblance to emo makeup).
Emospada. Hell, anything with 'emo' in it tends to work.
Alky Whora. Just because.
L-quiorra, because he looks and acts like L from Death Note.
he has a few nicknames, too. The most common one is "Shaq Fu", since he looks like Shaquille "Shaq" O'Neal. Alternatively, there's a variation of "Las Noches" that uses... a different n-word. Three guesses where that nickname came from (hint: it starts with "4" and ends with "chan").
Pumpkincar, because his released form looks like a giant pumpkin with eyes.
Kinky Boots: Her power from dirtiness Fullbring "Dirty Boots". You know you were thinking it.
Due to the likeness the Vandenreich have to Nazis, some people have started calling their operation in Hueco Mundo "The Hollowcaust".
The group itself have been called the "Vandenazis" or the "Vandershit."
Before his name was revealed, people called Yhwach "Buckbeard", after the title of the chapter he first showed up in. When his name was first revealed and sounded out, the fandom spelling was "Juhabach", resulting in his nickname morphing to "Bachbeard". He is also known as "Trollbach" or "Trollbeard" due to his plans and appearance screwing over everyone. Including the audience. Thanks to the Putting on the Reich themes he's inflicted on the Vandenreich and their Nazi-esque behaviour, he's also been called Quincy!Hitler.
The then unnamed Stern Ritters had a few nicknames to distinguish them. For example, Robert Accutrone was usually referred to as "OMQ" (Old Man Quincy) or Commissioner Gordon. Many fans also refer to him as Colonel Sanders.
Quammy" Driscoll, being a quincy that looked a lot like Yammy.
Bazz-B became known as either "Punkie" or "Punkdereich" thanks to his massive mohawk.
Haschwalth was known as "Blondie" prior to his name reveal due to a very early colour art reveal showing he had very long, blonde hair. He is also known as "Hashtag" or "Hashbrown". A 4chan-exclusive nickname "Lord Toofsy" originated from their use of re-captcha, an anti-spam tool, to nickname the Sternritter.
Cang Du: His physical appearance and weapon style led to the nicknames Quincy!Ezio or Quincy!Zuko.
"Bambi": Bambietta Basterbine. She went through a phase of being called "Miss Psycho Bitch" due to being so Ax-Crazy she slices a guy in half after inviting him to her room for sex. Later on, she became known as Zombietta due to fans theorising she'd become a zombie-puppet of Giselle's. Once she did, the name became very popular. She's also called Para Bailan La Bamba.
Candice Catnipp and Giselle Gewelle are known as "Panty and Stocking". Take a look at them to see why.
Meninas McAllon, due to being kind of an Identical Stranger to Orihime, has been called "Evilhime". She's also been called Melonas or [McMelons] for her huge bust size.
The girls are also known as "the Bambettes", as in the "Bambi and the Bambettes". It was based on the way the five girls were introduced and named, as per Kubo's love of music, to pun Bambietta's name in the style of the girlbands of the 1960s and '70s. Later, when it became clear Bambietta wasn't really the leader and the other four gained greater focus, "FemRitters" also became popular.
"Stinkmeaner" for Guenael Lee, because he looks kind of like that character from ''The Boondocks".
Until Pepe's name was revealed, he was mostly referred to by fans as Mr. GehGehGeh or Mr. Clownpants.
The moment Gerard Valkyrie turned up, his appearance immediately earned him the nicknames "Thor" or "Hawkman"
Lille Barro immediately earned the nickname "Bishop".
Goat-chin or Goat-face. This is based on his daughter Karin's nickname for him.
Papagami because he's the shinigami father of the main character
Arrowdad because entered the storyline arrows first and is the father of Uryuu, whose own nickname is "Arrowlad".
Arrowsad due to his Stepford Snarker status after his tragic backstory is revealed
From the Shusuke Amagai filler arc:
Big McLargeHuge, Thick McRunfast, Dirk Hardpec, etc. - The silent, tall and muscley bodyguard of the princess in the latest filler arc. All MST3K references.
Fillergami - derisive term for Shinigami created for the filler.
Captain Tuning Fork - Shusuke Amagai, the new filler-only captain, who wields a secondary weapon(a bakkoto). Three guesses what it looks like.
And for the Big Bad, Shusuke Amagai - Amagaizen for being a ripoff of Aizen. Their first names are even similar: Shusuke and Sosuke, as well as another "traitor", Kisuke.
Kamen Rider girl: The vizard Mashiro, because her entire design is an Homage to Kamen Rider.
Zhu Li: Lisa Yadomaru for her post-timeskip look which is near identical to Zhu Li's.
Namek - Soul Society, to people that view it as a Plot Tumor. In "honor" of the setting of the long, long Frieza arc in Dragon Ball Z. Usage: "Are they still on Namek?"
In the same vein, Hueco Mundo, better known as Mexico. "They still stuck in Mexico?" This lost some of its power when the story shifted to the Fake Karakura arc, although Ichigo and his friends are still inside "Mexico". Both have lost their luster due to the fact that the heroes are no longer in either dimensions in canon.
Karakuri Town - The fake Karakura town that Aizen invaded.
Plotkai - Any new plot power that appears. It's also applied to any Ass Pull on the show plot twist the user is dissatisfied.
Titty Kubo, Kubo Tits, Beaucoup Titties, Kubo Titties, etc - Tite Kubo. One look at Rangiku Matsumoto (or half the women in the cast) should tell you why.
RAEPFACE - Any Off Model and creepy-looking face, especially those made by Ikkaku◊ or Gin.
"THIS IS MY FILLER FACE! -insert goofy narmic face here-
The Bancouch - Aizen's ridiculously large couch.
The Couch Behind The Man
Soul Penis - the Zanpakutou. (Snickers were heard throughout the Soul Society when Ichigo was described as having a zanpakutou as long as he was tall and lacking in impulse control.)
And remember kids, never judge a man by the size of his... sword.
And then there's the manner◊ in which Mayuri carries around his zanpakutou.
Also, the manner of how Pesche pulls out his zanpakutou, Ultima.
De-armed - What Ichigo does to Yammy after the titanic arrancar obliterates Chad.
Also Diss-Armed, as a combination retaliation and insult.
Ichigo's Angels - The opening for seasons 11 and 12, using Shojo S by Scandal, specifically Matsumoto, Orihime, and Rukia. Many have also compared them to Bond girls since the OP is a lot like a typical opening for those movies.
Alternately, the "Hoagie". (Mmm, deli sandwiches of cosmic power...)
Pwning Festival: Aizen single handedly defeating the remaining Captains and Vizards.
Ever since Ichigo and some of his True Companions Chad, Orihime, Urahara, Nel and Pesche took off to Hueco Mundo to fight the Vandereich there, their journey has been renamed as "The Hueco Mundo Road Trip"