Over the long history of the Teenage Mutant Ninja Turtles, some characters have become inextricable to the lore. Fans wouldn't accept a Turtles property without April or Shredder, even though they're radically different in almost every appearance. Some characters aren't quite as important, but they show up often and make the franchise a bit more exciting. Look at Leatherhead, a mutant alligator whose been around since the beginning.