So far, so good! I like that she has a nickname, it adds humanity.
This is good, but also not going to work. Sorry. Leviathan have almost no role in this role play, but a possession could work. It could have been angels, or demons. She also doesn't need to know what really caused the problem. As far as the 3 years ago, that's good. Overall, it is solid and makes sense, just change it from leviathan (maybe werewolves?) to pretty much anything else and it's good. Actually, f**k it, I'll add the leviathan in later. However, (*spolier alert for the TV series!!!*) the leviathan don't become well known for a long time, so don't expect her to know any time soon. It's going to be a dead end, and the immediate bad guys are actually working to hide the fact from her. This is good, and way fitting as an explanation for why someone would be a hunter.
This is going to start the night of their graduation, so she has finished up high school. No worries though. I like this part a lot, actually, she is shown as strong but willing to restrain herself. It also shows the connection between herself and Gwen, which is imperative and stronger than any other bond in the RP.
This is good, I like it! However, I need you to re-read the rules and edit your CS accordingly (*cough* rule 10 *cough*) before I can approve it. Once you do that, I can guarantee you'll be approved.