My idea is kind of a mix of Constantine, Venom, and Ben from umbrella academy, with a big Lovecraft vibe. Constantine is personality and knowledge of mythos lore, but possessed by a monster that, while it helps him fight bad guys, he has to keep pacified or else it will slaughter people. (Venom vibes on the inner beast thing). Depending on you limitations, it could be possible that the thing repairs him "save the host" or it can be set free if he dies, so wouldn't necessarily heal him. Was going to have him play the fiddle to pacify the beast when it gets too out of control. But the thing wants to eat, so the man decides if he goes out and fights bad guys, he can feed the bad guys to the thing to make it happy. 

I know this is your first time running, so I can totally make a new idea if this is all too much... my imagination ran away with me today. Lol.