Splinter

Master Splinter or simply Splinter is a mutant rat. He trained all four of the Turtles.

Mirage
In the original comics, Splinter was the pet rat of a ninja named Hamato Yoshi in Japan. Intelligent for his species, Splinter was able to learn his master's art by mimicking his movements while he practiced. Yoshi became embroiled in a dispute with a fellow ninja by the name of Oroku Nagi and was eventually murdered by Nagi's brother Oroku Saki.

Without a home, Splinter was forced to run away and live in New York City's sewers. Due to a traffic accident, four baby turtles and a canister of radioactive slime were sent down into the sewer, where they mutated both Splinter and the turtles. Splinter named the four turtles Leonardo, Donatello, Raphael, and Michelangelo (after a book on Renaissance artists that he found in a storm drain) and trained them in the arts of ninjutsu, while still raising them as his own sons. Splinter died of old age in Teenage Mutant Ninja Turtles (vol. 4) #10.

1987 series and Archie comics
In the 1987 series, Splinter was changed to be Yoshi himself, and was framed by his rival, Oroku Saki (who would eventually turn into his future arch-enemy, The Shredder), for trying to murder their common dojo master. Being unable to prove his innocence, he ran away to New York, where he lived as a hermit in its sewers and befriended the rats.

One day, he came across four baby turtles which were accidentally dropped by a boy through a grate of the sewers. He kept them as pets and treated them like his sons. But later, he found the turtles venturing near broken barrels that were oozing glowing pink chemical liquid. As he tried to recover them from danger and attempted to clean them with his bare hands, they were all affected by the chemical which was a special mutagen. This mutagen transformed any living being into a mutant whose body became mixed with the DNA of any other living being it was last in contact with. The turtles, being touched by Hamato Yoshi, turned into young anthropomorphic humanoid turtles. Hamato himself, having lived in rat-infested sewers for so long, turned into an anthropomorphic humanoid rat.

Since then, Hamato brought the turtles up, and gave them the names of his favorite Renaissance artists: Leonardo, Donatello, Raphael and Michelangelo, who were to be distinguished by their personality and headband color. He also taught them the art of ninjutsu in order to protect themselves and help to confront the Shredder and his hordes from the Foot Clan.

Hamato was given the nickname of "Splinter" by his adoptive sons, due to the way he easily splinters his enemies. (In the cartoon, the nickname comes from his proficiency at breaking wooden boards)

He is portrayed as a light brown-grey rat in a pink kimono, wearing white bands on his feet.

In the Archie comic series Teenage Mutant Ninja Turtles Adventures Splinter is the same as in the 1987 series, because the first issues of the Archie comics follow the cartoon.

In Japanese dub of the 1987 series, his true name is "Takeshi Yoshihama:タケシ・ヨシハマ(吉浜武士)".

Movies
The movie origin followed more closely to the original Mirage Comics then the Teenage Mutant Ninja Turtles (1987 TV series). Splinter was the pet rat of Hamato Yoshi in Japan who followed the teachings of his owner from his cage. When forced to America and after his master's death, Splinter found himself in the sewers of New York where he found four turtles.

One morning Splinter awoke to find the turtles playing in a glowing green goo spilling out of a broken canister. Soon, he noticed that not only were the turtles changing, so was he. Eventually, he grew to his full mutated form and began to teach his adoptive sons the ways of honor and the path of the ninja.

Where in the first film Splinter was a dignified father figure for the Turtles, in the subsequent sequels he became more of the "hip" elderly person offering more and more one-liners with each sequel.

Finally, in the 2007 film he had once again reverted back to his origins of the dignified master, and father with only a handful of humorous scenes in his role.

Image Comics
In the Image Comics Splinter turns into a giant bat-mutant.

The Next Mutation
Splinter is a more elderly rat than seen in most other incarnations and is the adoptive father to the 4 18-year-old turtles. During his spare time, he is seen meditating in a supernatural dimension known as the realm of dreams and plays chess with a blind man called Andre, whom he invites to live with him and the turtles, when Andre is evicted from his apartment. Splinter is also an old friend of a mystical Shinobi Shaman named Chung Li, who warns him of the Dragonlord's plan and demands of Splinter's release when he is kidnapped, unfortunately at the cost of Li's own life. Splinter remains in a coma-like state for a few episodes, until the turtles free him from the Dragonlord's domain. He then reveals that he knew all along about the existence of the fifth turtle Mei Pei Chi ( better known as Venus De Milo) and welcomes her into their family, as his daughter. He encourages Venus to no longer mourn Chung Li's passing as he 'has gone on to a better place'. Splinter still shows formidibility and exceptional skill in combat, as he is seen easily defeating entire hordes of Foot soldiers and Dragon warriors. The only warrior shown better to him in combat is the fierce Dragonlord himself. Splinter still shows a sense of humour, often making jokes and wise cracks like his students when battling or simply in the comfort of his own home. He was going to be shockingly killed off in the second season(most likely by the Dragonlord), leaving the turtles to learn how to fight without their master's guidance or prescence, but it was cancelled before this could happen.

2003 series
Splinter started his life as an ordinary rat in Japan who found his way into a home owned by the Ancient One. When he was discovered, his life was spared by Tang Shen, the Ancient One's adopted daughter, who took him as a pet. As in previous versions, Tang Shen became tragically caught up between Hamato Yoshi and a jealous rival, this time Yukio Mashimi, a childhood friend of Yoshi's. The two of them eventually joined the Utroms and became Guardians. But Mashimi's jealousy of Yoshi, both for his advancement as a Guardian, and the love of Tang Shen, led him to murder Shen and betray the Utroms to The Shredder. Yoshi killed Mashimi in revenge and went to America with the Utroms, taking Tang Shen's pet with him, and giving him the name Splinter. Later, Yoshi would be tracked down and killed by the Shredder, who tried to force him to reveal the Utrom's location. In this continuity, it was Hun that Splinter scratched as his master Yoshi was attacked, and not the Shredder.

He is portrayed as a dark gray rat in monks robes. He is also referred to during the Battle Nexus story arc as 'Hamato Splinter', indicating that in this series, Hamato was the surname, not the given name, of Splinter's master. Splinter also had an opponent in the Battle Nexus named Drako, who would prove to be a dangerous adversary of the Turtles.

Lost Season
Splinter appears most of the time alongside the Ancient One expressing his deepest concern for the safety of his sons and for the harsh teaching methods of the Ninja Tribunal.

Fast Forward
Splinter has been seen in the Fast Forward season continuously teaching his sons' and Cody the ancient ways of ninjutsu and despite how old fasioned he's shown to be in previous seasons he too adjusted to living in the year 2105 fighting off any evil that came his way alongside his sons

Back to the Sewer
Splinter didn't make much of a physical appearance in Back to the Sewer however he did appear as small data fragments scattered across Cyberspace thanks to Viral and the only times he was every physically shown was during the first episode of BTTS called "Tempus Fugit" when he was first scattered into a million data pieces and the last episode called "Wedding Bells and Bytes" when he was returned to the real world and attended April and Casey's wedding ceremony which was abruptly interrupted by the appearance of "The Cyber Shredder"

Video Games
In most games Splinter is an NPC that often has to be rescued. He actually gets to be a playable character in the 2003 game, Teenage Mutant Ninja Turtles 2: Battle Nexus, and TMNT: Mutant Melee. Also, portrayed in the first TMNT he is a black guy.