Splinter (Hamoto Yoshi) was a ninja master who was mutated into a rat by some radioactive mutagen. Now, he is the mentor and trainer of the Teenage Mutant Ninja Turtles. He is never seen in the series, but was heavily referenced in the episode Truth, Justice, and What?.
When Robin and Donatello are fighting with their staffs and Donatello hides in his shell, a small rat wearing ninja garb emerges from the shell and kicks Robin in the face. After the Teen Titans beat the TMNT, they have a small pizza celebration, with Beast Boy transformed into a miniature likeness of Splinter.