There is a wonderful ensemble of characters in Teenage Mutant Ninja Turtles, because they are all relevant pieces of one streamlined story. In addition to the four Turtles, we have April ONeil, Splinter, Casey Jones, the Shredder, Tatsu, Danny, Charles, and Chief Sterns. The Turtles could have easily fallen through the cracks of their own movie like the Transformers would do in later years, but because the filmmakers respected each characters place to appropriately serve the story, each character has his or her moments to shine. The Turtles remain the first priority, and Splinter and April interact with them appropriately. Shredder has enough time to simmer, be a grand presence, and plot. Casey Jones is a foil who could have easily been saved for a sequel, but he doesnt weigh the story down at all, because the dimension he adds is sprinkled on just enough, and fits in with the original mythology of the Turtles.
Chief Sterns is fun whenever he appears and ultimately serves the stories of April, Charles, and the Shredder. The scene of Tatsu going berserk on Foot Soldiers shouldnt be relevant. Tatsu is an original character and has a decent amount of screen time despite the probability that the films original audiences werent really paying to see him. But his conflict gives his character some good depthwithout overtaking the Shredder in presence. Danny and Charles had absolutely no reason to be the least bit absorbing. The whiny everyman kid learning a lesson with his single parent is a cringeworthy staple of most mainstream kids movies. But Danny is played with reality and is respected as a person, and his part brings home the larger themes of the movie (more on that later). And no one paid to see the Shredders Lost Boys either, but they bring their own interesting arc to the table that in no way feels excessive. Its sort of like caring about Gothams fate in The Dark Knight Trilogy.
But dont stop there, Platinum Dunes! As X-Men: The Last Stand so impeccably reminds everyone, solidly-founded characters mean nothing if filmmakers dont find interesting things for them to do, and part of that comes from the places where they interact. Lets take a step further in.