Hamato Yoshi AKA Splinter has always been the "Master" of the turtles, their martial arts teacher, leader, and mentor. In IDW TMNT, he's actually their biological father, reincarnated (check out the TMNT micro-series, or the 2016 FCBD issue). That helps give the story more depth and can sort of give validation, in some minds, for how all of this happens.
