In the 2012 Teenage Mutant Ninja Turtles animated series, Splinter is human before mutation.
The series identifies Splinter’s human name as Hamato Yoshi. After exposure to the mutagen known as ooze, he transforms into a humanoid rat. This origin makes the 2012 version distinct from stories in which Splinter begins as an ordinary rat who mutates alongside the turtles.
Hamato Yoshi’s history is tied to Japan and to his long-running conflict with Oroku Saki, who becomes the Shredder. The rivalry gives Splinter personal stakes beyond his role as the turtles’ martial-arts teacher. He raises the four turtles in the sewers and trains them to survive.
A common mix-up comes from the 1987 cartoon and several other adaptations, where Splinter was formerly Hamato Yoshi but became a rat through a different transformation story. The 2012 series keeps the human identity while presenting its own mutation details.