@unicorgiDelevope your killer as if you were making a horror movie villain. We may know a bit about their past. But generally as a rule, our actual understanding of them outside of their horrorific nature and cause is not normally a lot. They can be a slasher...or maybe a spirit like the grudge.
You really have full reign on how developed you want them to be. You don't have to follow my suggestions, it's really up to you. But your host will rarely-if ever- appear out of their domain yet alone as anything but a predator on the prowl-or whatever your killer does-
Perk denote your powers and your interaction with the mechanics.
Overwhelming Strength: for example. Basically translate to-if some MP character blocks a door with a sofa. Instead of being told you're ramming the door for the next 2 minutes in RP world as you watch MP post how they run away before you can finally post a reaction. You just make a post detailing how you just rip the door off and push the sofa out the way but you make less ground on them because you just had to move a bunch of shit.