Shredder developed a plan to defeat the Turtles by mutating members of this street gang, so that they would have abilities parallel to the Turtles'. Bebop and Rocksteady both volunteered to undergo the procedure (though neither was particularly aware of what it would entail) with the promise that it would let them get revenge on the Turtles. Bebop was mutated into a human warthog, and Rocksteady into a rhinoceros. However, though the transformation did make them larger and stronger, they remained incompetent simpletons, and completely inept at stopping the Turtles or carrying out Shredder's plans. For most of the series they were employed for comic relief purposes. In one episode of the series, Bebop was shown to have kept a pet turtle, which got mutated into the evil turtle Slash.

The son of the Battle Nexus Daimyo, the Ultimate Ninja first appeared when he came to Earth to fight Oroku Saki, the Shredder. However, he was cheated of this battle after learning that the Shredder had seemingly met his death at the hands of Leonardo. In response, he challenged Leonardo to a duel, and the two fought to the death. Where Leonardo fought with honor, the Ultimate Ninja did not, and even attempted to kill Leonardo after the fight ended. The Ultimate Damiyo made his son ask for the Turtles' forgiveness, an act that the Ultimate Ninja considered beneath him and humiliating.

Donatello's bandana is typically portrayed as purple, his favorite color and his weapon is a bo, which he sometimes uses as a tool or a walking stick. Donatello is the whiz kid of the TMNT, being adept with machines, though other traits of his personality vary depending on the exact media in question. He is named after Donatello, the famous Renaissance sculptor.

Shredder's physical appearance remains fairly consistent in all incarnations of the character. As Oroku Saki, he looks like a physically-fit, middle aged Japanese man. But most frequently, he is seen in the Shredder persona, wearing a suit of armor vaguely based on samurai armor, modified to be less cumbersome yet more menacing. The armor consists of spike-covered metal plaques on his shoulders, forearms, hands, and shins; underneath it he wears a purple robe that variously appears to be simple fabric or a form of chainmail. He also wears a metal helmet with a trident-shaped ornament on top, and a metal mask that covers his face, leaving only his eyes visible. Originally going to be called "Grate Man", while creating Shredder, Kevin Eastman got the idea for Shredder's armour from large trapizoid shaped graters, thus Eastman drew one on his forearm, gripping the handle from inside the grater. He then said, "Could you imagine a character with weapons on his arms like this?", and thus Shredder was born.

Raphael's bandana is red (his favorite color), and he carries two sai. His personality is usually aggressive or sullen, and he tends to be the "rebel." He is named after Raphael Sanzio, the Renaissance painter.

Raphael's anger is not always fully explored, although in certain incarnations of the TMNT, it seems his anger stems from realizing, more than his brothers do, that they are the only creatures of their kind and are ultimately alone. Furthermore, he is often shown as feeling jealous of his brother Leonardo, who leads the TMNT. He is generally the most likely to experience extremes of emotion.

Leonardo's bandanna is blue, and his weapons are two katana. He is named after Leonardo da Vinci, the Renaissance artist and inventor.

Leonardo takes after his sensei, Splinter, in taking Ninjutsu very seriously. Of the four turtles, he is the closest to Splinter and spends much of his free time in practice and meditation. He has a strong sense of honor, and is a strict follower of the Bushido code. He tries to keep his brothers in line and often has the leading role in the characters' adventures.

A character created for the cartoon to replace Oroku Nagi, Yukio Mashimi was the life-long friend of Hamato Yoshi. As two orphans living on the streets, Yoshi and Mashimi are taken into the home of the Ancient One where he trains them in the art of ninjitsu. It is here that Yoshi and Mashimi fall in love with Teng Shen, another orphan cared for by the Ancient One. Yoshi and Mashimi eventually become Guardians against the wishes of the Ancient One. Mashimi's jealousy begins to grow as he sees Yoshi ascension within the Guardians along with the fact that Shen loves Yoshi. Mashimi eventually snaps and murders Tang Shen. He then joins the Foot Clan and leads the Shredder to the Guardians' hideout. Seeking revenge, Yoshi attacks the Foot Headquarters and faces Mashimi in a duel. After suffering a fatal wound, Mashimi asks Yoshi for forgiveness and dies thereafter.

Splinter is the master and father figure of the Teenage Mutant Ninja Turtles. In all versions of the story, he is a mutant rat, but his origins often vary depending on the version. In the original comic, Splinter was the pet rat of a ninja named Hamato Yoshi. Exceptionally intelligent for his species, Splinter was able to learn his master's art by mimicking his movements while he practiced.

Yoshi eventually was embroiled in a dispute with a fellow ninja by the name of Oroku Nagi over the love of a woman named Tang Shen. When the woman favored Yoshi, Nagi grew angry and beat her. Yoshi came across them, and slew Nagi in his rage.

The pair escaped to New York City, but they were tracked down and Yoshi was slain by Nagi's younger brother, Saki, who had become The Shredder.

Without a home, Splinter eventually came to live in the city's sewers. One day, he saw a canister of chemicals go flying from a truck. The canister struck a boy in the face after he saved a blind man from being hit by the truck (a reference to the superhero Daredevil), and then struck and shattered a bowl full of four baby turtles carried by a young boy. The now broken canister and turtles fell into the sewers, where Splinter rescued them, also exposing himself to the ooze that had covered the turtles.

In time, the ooze mutated both Splinter and the turtles, increasing their size and intelligence, and making them more humanoid. Eventually, Splinter named the four turtles Leonardo, Donatello, Raphael, and Michelangelo (after his favorite Renaissance artists) and raised them as his own sons, but also trained them in the arts of ninjutsu, so that they may one day avenge the death of Hamato Yoshi and slay the Shredder, who had amassed a powerful organization in the New York underworld.

Krang is an extremely intelligent warlord from Dimension X. He somehow took command of an army of Rock Soldiers under the leadership of General Traag, and stole the Technodrome, a powerful battle fortress, from its creator Drakus. Something then happened, that caused Krang to be stripped of his body and confined to a brain-like form, and to be exiled from Dimension X to Earth, along with his army and the Technodrome.

While on Earth, Krang allied himself with the Shredder, who, along with his robotic Foot Soldier army, moved into the Technodrome. In exchange, the Shredder had to design and build a new body for Krang, a human-shaped exo-suit. Shredder lived up to his part of the bargain in the season 1 episode The Shredder is Splintered, in no small part because he was unable to deal with the Turtles and needed Krang's help. Indeed, in the season 3 episode Shredderville, the Turtles visit/dream a parallel world in which they never lived, and Shredder had no problem taking over the world. In this world, Shredder abandoned Krang after his conquest was complete, leaving him with no body and a heavily-damaged Technodrome.

Krang's ultimate goal is to take over the Earth; it probably only became his objective after he was exiled on the Earth, but this point is never made clear. Every plan Krang conceives is either aimed at that goal, or towards the short-term objective of powering-up the Technodrome. He does not share Shredder's obsession with the Turtles; while Shredder sees them as mortal enemies, Krang seems to regard them more like annoyances to be destroyed when they interfere in his plans.

Counting from the first meeting between the Turtles and Shredder and Krang, Krang spent eight years in his Technodrome, either somewhere on Earth or in Dimension X, scheming to power up his battle fortress and take over the Earth. In the season 8 episode Turtle Trek, the Turtles destroy the engines of the Technodrome, trapping it and its inhabitants in Dimension X for good and putting an end to Krang's plans.

Krang spent the next two years in Dimension X, until he was contacted by Dregg (The Power of Three). Dregg arranged for him and Shredder to come back to Earth, to help him fight the Turtles. However, Dregg betrays them, and drains Krang's intelligence. Shredder escapes and restores Krang (A Turtle in Time), but Dregg captures them again. Finally, the Turtles spoil his plan and transport Shredder and Krang back to Dimension X (Turtles to the Second Power).

In the series finale Divide and Conquer, the Turtles return to the Technodrome to take Krang's exosuit, which they need to fight Dregg. Krang is nowhere to be seen. But it is assumed that he is still somewhere in Dimension X.

In the Archie Comics series, Krang was depicted as far more evil and wicked than in the cartoon. He was said to have the blood of whole races on his hands.

Arnold "Casey" Jones (sometimes spelled K.C. Jones), like the turtles, is a self appointed vigilante in his community, who uses dual baseball bats as well as golf clubs, cricket bats, and hockey sticks to pummel the living daylights out of would-be criminals. He stands at a full 6' 2", and dons a hockey mask (giving him an undeniable visual parallel to Jason Voorhees, albeit of a distinctly different design), shoulder pads, cut-off biking gloves, and carries his array of weapons in a golf bag he slings over his shoulder. Clad in his fighting gear, Casey wanders the streets of Manhattan apprehending hoodlums that terrorize his neighborhood in a fashion not dissimilar to Marvel's Punisher, although without usually employing the latter's deadly force. Casey Jones is a master of stealth, as he has never been caught by the NYPD.

In the original TMNT animated series, April O'Neil (whose middle name was purported to be Harriet) was a television reporter for Channel 6 News. She was employed by Burne Thompson, though due to her headstrong nature and passion for her work, she expressed frequent disagreement with the assignments he gave her. She also butted heads on a regular basis with Vernon Fenwick, the director/camera operator whose enormous ego compelled him to scoop April on her stories whenever possible. April was best friends with Irma, the secretary at Channel 6. Though it remains unclear whether this was required work attire or her particular fashion sense, April could usually be found wearing a distinctive yellow jumpsuit. She lived in an apartment in New York City, though during the course of the series, she was forced to relocate several times due to a variety of indirectly Turtle-related mishaps.

In 1987, having been employed at Channel 6 for less than a year, April was reporting on a series of thefts of high-tech scientific equipment, apparently by a ninja (ostensibly components Shredder was securing for use in the Technodrome), when she came under attack by a gang of punks. Thinking quickly, she managed to squeeze into a storm drain and ran from the mob until she hit a dead end. As it happens, the Ninja Turtles were nearby and soundly beat the punks. They took her back to their sewer lair for rest and recovery where they explained their origins to her. At first, she believed them to be responsible for the equipment thefts, but they agreed to provide her with the true culprits if she provided her assistance. April quickly became the Turtles' link to the outside world, since their unusual appearance effectively precluded them from functioning aboveground without some sort of disguise.

April's friendship with the Turtles resulted in the opportunity to film exclusive footage of their encounters with Shredder, but despite her intimate knowledge of the details of their lives, she tended to keep her professional news reports about them impersonal and mysterious. She was a vocal champion of their cause, despite the opinion of the general populace (an opinion shared by Burne Thompson) that the Turtles were a menace to the city. Through no fault of her own, April was frequently captured by Shredder, quite often as bait in order to lure the Turtles out of hiding in order to unleash his latest attempt at destruction upon them. April's innate curiosity and determination placed her in danger on several occasions as well. The Turtles usually waited until the last possible second to rescue her, but they always came through in the end. Her most important contribution to the Turtles was her experience in doing research on the Channel 6 computers and alerting the Turtles to trouble and possible case leads.

Like Lois Lane before her, April was normally portrayed as a capable woman, but also filled the role of the typical plot device of damsel-in-distress. Her obsession with "getting a story" led to numerous instances of poor judgement which often resulted in her getting captured for the Turtles to save.

Hun’s criminal career began at a young age; during his teen years, he was already a prominent member of the Purple Dragons street gang, and was responsible for the burning down of Arnold Casey Jones’ (Casey’s father) store, as well as his eventual death. It is during this time that the the Shredder, realizing his potential, takes him under his wing, training him as his protégé. At the beginning of the series, Hun is second-in-command of the New York faction of the Foot, as well as leader of the Purple Dragons (although he leaves its day-to-day operations to people like Dragonface). During the City at War arc, where New York’s underworld is fractured due to The Shredder’s presumed demise, he leaves the Foot and takes direct control of the Purple Dragons, until he is informed of his master’s true fate and returns to the Foot. A more definitive split occurs in season 4, after the Shredder’s true identity as an Utrom is revealed and he is exiled: with the Shredder gone, Hun turns to the Purple Dragons, successfully remaking them into a Foot-like organization.

Hun is a lethal fighter, surprisingly agile and skilled with a multitude of weapons (although he prefers to use his fists). He is also quite smart, as evidenced by his success in running the new Purple Dragons. However, his power level, as it is, has been inconsistently portrayed throughout the series: although sometimes he is able to hold his own against all four turtles, at others, he is able to be defeated by a single fighter. Generally, he is defeated more easily the later he appears in the series, presumabely due to the turtle's increasing skills.

As a youth growing up in Casey Jones' neighborhood, Angel comes under the influence of a street gang known as the Purple Dragons. She quickly becomes involved in thefts as part of her initiation and soon finds herself fighting in a wrestling ring to become a full-fledged member. Casey comes to her aid when he and the Turtles infiltrate a Purple Dragon hideout and pull Angel off her path of delinquency. Angel then becomes a close friend of the Turtles.

Angel returns to the Turtles when her older brother goes missing during a construction job, which leads the Turtles to the Volpehart Building and an extraterrestrial creature that forces them to confront their own fear.

Angel also appeared at the Turtles' lair for Christmas. She and Raph tried to get April under the mistle toe for Casey.

Angel's most recent relation to the Turtles was serving as a message carrier after their lair was destroyed by Karai and the Foot.

Dirty penny was a dirty High Elf killed by the Turtles in game 9. Unfrotunately he lost his dirtiness after the transition to Undead!

The Doors was a Skeleton for Music and Death, who was killed in the Turtles' third match.

John Jules was a Bull Centaur for Laundry Boys, who was killed by Rocksteady in the Turtles' first ever match.