Casey Jones is a vigilante anti-hero from the Teenage Mutant Ninja Turtles series, appearing in nearly every incarnation of the franchise. He's a fan favorite, having the same general appeal as Batman, The Punisher, or Todd McFarlane's Spawn.
The anti-hero is a very popular archetype, possibly because it presents to the audience a vicarious wish fulfillment fantasy. We all wish we could stand up to the bullies and fight for justice, but who wants to be constrained by the sort of restrictive morality the standard hero must abide by? How often has the hero had the villain defeated, only to show mercy at the exact wrong time, allowing the villain to gain the upper hand? We know that when Spider-man carts Doctor Octopus off to jail, he's just going to break out again. We know that Superman shouldn't hand over the last microchip Lex Luthor needs to build his superweapon just because he's holding Lois Lane hostage.
Wouldn't it be so much more satisfying to break the villain's arms so he can't cause trouble anymore? Wouldn't the world benefit from the death of one criminal if it means he can never hurt anyone again? On the other hand, is it worth sinking to the villain's level, using the same tactics he would? Would it even be effective to commit violence to try to stop violence?
Do the ends justify the means?
Casey Jones thinks so. Making his first appearance in the original Mirage TMNT comics, Casey Jones is the very definition of the vigilante anti-hero. Casey apparently lived alone in a small apartment, spending his free time lifting weights while watching cop shows, Clint Eastwood movies, and the local news. On three TV sets at once. His view of the world became a bit warped, and he decided that the police were too restrained by such petty details as miranda rights and due process of law. Donning a hockey mask, sports pads, and a golf bag full of blunt instruments, Casey hit the streets to punish the criminal element the way he felt it needed to be done.
Unfortunately Casey Jones has no sense of scale. Be ye purse snatcher or rapist, petty thug or criminal mastermind, there is only one level of law enforcement to Casey's way of thinking. He's out to punish injustice, and that involves breaking some bones. Being tough on crime means hurting the criminal so badly, he'll be afraid to ever commit crimes again.
Original Comics
Raphael was the first of the Turtles to meet Casey in the comics. He had lost his temper sparring with Michaelangelo and needed to get out for a bit to clear his head, when he ran across Casey beating up some muggers in Central Park. And by beating up, I mean he had already defeated them, and was continuing to pummel their battered and broken bodies, ignoring their pleas for mercy.
Being a traditional hero with restraint and a solid moral code, Raphael couldn't let this go, and confronted Casey with his sense of ethics. Casey didn't want to hear it, and it escalated into a fight, in which Casey was actually doing pretty well until Raphael finally calmed down enough to use his ninja skills effectively. The fight ended with the two gaining a respect for each other, and Raphael learning an important lesson about his temper and where it could lead him.
The Cartoon
In the 1980s cartoon, Casey was much the same, but played for laughs rather than as a tragically flawed hero. The major changes to his character were that he switched from fighting injustice to fighting lawbreakers, he spoke in a Dirty Harry voice, and he never, ever took off his hockey mask. Again, the Turtles were forced to step in to help curb his extremist views on fighting crime, although this time it was because the media was confusing Casey's crimefighting tactics with the Turtles, and giving them a bad name.
The Movies
Elias Koteas played Casey Jones in the first Teenage Mutant Ninja Turtles movie, introduced in a first meeting with Raphael very similar to its counterpart in the comics. However, while in the comics Casey and Raph ended their fight as good friends, off to stop another mugging together, in the movie Casey got away. He returned just in time to give a badly needed assist during a fight in April O'Neil's second hand store when the Turtles were outnumbered by the Foot Clan and down one team member (Raphael having been ambushed earlier). Although he stayed to help out the Turtles for the rest of the movie, going so far as to beat Shredder's right hand man in combat, he didn't make an appearance in the sequel and only had a minor role in part three.