Splinter faces Shredder after he defeats Leatherhead. Unwilling to fight his daughter, Splinter finally decides to retreat, declining to tell his sons what he has discovered - He is mostly in favor of allowing them to celebrate their supposed victory over the Kraang. Initially evenly matched, Splinter begins to gain an advantage when he starts to rely on his abilities as a rat, partially breaking one of Shredder's blades in his teeth and running on all fours to evade his attacks, but Splinter is then knocked off-balance emotionally when he realizes that Karai is his own daughter, Miwa, raised to hate him and believe that he was responsible for the death of her mother. Although initially amused at the discovery of Hamato Yoshi being a Mutant rat, Shredder quickly engages him in battle, angrily proclaiming that Hamato stole everything from him, while Splinter counters that Tang Shen was never, ever Shredder's. While the Turtles are forced to confront the Kraang and rescue April, Shredder uses April's abduction to utterly trick Splinter into confronting him.
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Splinter looks sadly at his corrupted daughter. Splinter tells of his rivalry with The Shredder to a curious Leonardo, who wants to know if he can truly handle being the leader of his brothers. As a celebration of the Turtles' fifteenth 'birthday', Splinter, though reluctantly, allows them to explore the wondrous streets of New York City. It is revealed that he had also kept the canister of Mutagen that mutated them all as a charming souvenir. Splinter is seen telling the pivotal story of his mutation to the Turtles. When they turned fifteen, Splinter, though reluctantly, allowed them to leave the lair for the very first time. Taking them in as his sons and students, Splinter raised the turtles and trained them in the ways of Ninjistu. For the following fifteen years, Splinter and the Turtles lived in hiding in the sewers under New York City. He became a giant rat, and the turtles became humanoids. Splinter and his newly bought pet turtles were all covered in ooze and mutated instantly. A fight ensued, and while Splinter was able to defeat the men (who were actually Kraang droids in disguise), the canister was broken. As Splinter watched the first man exchanging a strange canister of ooze with another, he stepped onto a passing rat that gave away his position. As he was walking down the street, he bumped into a suspicious man, whom he curiously followed into an alley way.
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One fateful day, he bought four baby turtles at a pet store. Under the alias "Splinter", he came to live in New York City. Devastated, Hamato gave up his old name and left Japan for an eternity. While Yoshi survived the fire, Tang Shen did not, and their daughter, Miwa, had supposedly died as well. During the battle, the house caught fire and Saki mercilessly left Yoshi to die. One night, Saki, mad with jealousy and hatred, attacked Yoshi in his home, intending to kill him. Eventually, Tang Shen married Yoshi and gave birth to their daughter, Miwa. However, both men fell in love at the same time with the same woman, Tang Shen, which ended up sparking a bitter rivalry between them. Originally a human named Hamato Yoshi, Splinter lived in Japan and studied ninjutsu alongside his best friend, Oroku Saki (later known as The Shredder).