Master Splinter aka Master Sphincter is the master of the Teenage Mutant Ninja Turtles. He lives underground with his mutant offspring as result of coming in contact with toxic waste and sewage. Now he trains his turtle sons how to throw down so they don't take shit from evil assholes.
In the past, he was an ordinary rat who learned how to fight by watching his owner practice his Tae-Bo workouts on TV. He stayed with his owned until Shredder, a rival master from Mount Fugi challenged him to a duel and killed him. With no owner, Splinter became a rat living on the streets, living off whatever he could find, being a rat and all. Eventually, he bummed around like a hippy of monk until he got washed down a sewer, where he found some sea turtles. The sewage was mixed with toxic waste which turned them into freakish mutants, moving their evolution rate up to humanoid level. As they developed, Splinter, who learned the fighting arts, taught them to throw down for self-defense. At first, Splinter wanted them to be badass like some UFC fighters but he decided to throw in stuff like being cool with other people is good and the shadows are where its at. Together, their tough mutants, their cool and all but they'll f u up if you cross them. But that's what you get when Wu-Ta er... Splinter raised you.