Here's my guess: Thomas/Shawn would be too obvious. It can't be any of the victims or her friends cause they're all dead but Chris. The families of the victims didn't know (dangitiforgethowtospellhername) Casey? Kasey? Anyways.
Theory 1: Chris is the killer. He has the MPD disorder thing and he's forgetting he killed people's.
Theory 2: Chris is the killer. He has the MPD disorder thing and Kasey herself was one of his personalities. He is imagining all of this story from a padded room.
Theory 3: It is a character we do not yet know of.
This is a great story, I love it. Killer Inside and Killer Reborn are amazing, even though you're only six posts into KI, I know it'll be amazing.