I could write a short essay on why Jason is one of the most fascinating slasher characters to ever grace a film. Not because of his character, but how the character evolved on film. It's Friday the 13th, so lets condense that down to a short, digestible list of fun stuff about Jason and Friday the 13th.
**SPOILERS TO A 35+ YEAR OLD FILM****
1. In the first Friday the 13th Jason kills NO ONE. He's not the killer in the first film. His mother is, which was intended as just a spin on the Psycho story. You don't discover it's the mom till the end. They wanted a double twister, though, so at the VERY end it turns out the boy (Jason) was alive the whole time.
2. Jason was never intended to continue the Friday the 13th series. None of the original characters were. It was the audiences strong reaction to the end of the first film that earned his spot in the 2nd movie.
3. Jason never puts on a hockey mask until the 3rd movie. Even that's about half way through. The stunt man that played him said he felt so natural in it, that even the director commented on how much more "in character" the actor suddenly became.
4. Certainly up for debate, but nothing supernatural happens in the Friday the 13th movies until the 5th one, when Jason truly returns from the dead.
5. Jason Voorhees is the only slasher film killer I can think of that was truly brought to life on screen. What I mean by that is, for example, Michael Myers was already a legend at the start of the first Halloween. Same with Freddy. Their back story had to be told to progress the story of the film. Jason isn't a legend until he earns it, right in front of the audience.