Jeff has a conflict of identity. He pushes his "character" of the flamboyant, charismatic and intimidating serial killer hard; on the outside he is vain and meanspirited, territorial and highly defensive. He gets a kick out of taunting and playing with others, and professing to not care about who he hurts at all.

On the inside, he's kind of losing himself. He's deeply insecure and harbors an intense inferiority complex to others and dislikes himself quite a bit. He feels empty underneath his 'masks', and while he pretends to not care, in truth he struggles actively with not hurting anyone and is a little scared of himself. He's the guy that has the biggest foothold in his old life. He's got some bad complexes with trying to impress and entertain others, but he's genuinely a people person somewhere in there. Give him an A, he's trying.

Jeff is a dumbass. Skip this section.

...Actually I don't know how much higher in ability he is to the average human. He can take a lot of hits, and I think if he were incapacitated his creature puppeteer could steer and heal his body. I'll have to chew on it.

Jeff grew up in an unremarkable middle class home and spent his teen years moving in the 'popular' social circles, drifting away from his old friend whom he knew since they were kids. He became one of the cool kids, and while he tried to keep up, found that the grass definitely wasn't greener on the other side. He graduated and mysteriously disappeared, leaving his home and family in shambles.
In truth, he became the puppet of a nameless creature using him to gain power. He spends the next few years as its lackey, only to bump into his old friend again. Now, the tables have turned; she's everything he wishes he was -- smart, confident, has her shit together.
That's not all, though. A familiar tall man has its eye on Jeff as its new proxy, and now he'll have to find a way both to protect Hazel from it and get out of this play whose acts go on and on and on.