Might be a little simple but you could go with Force. Like life force and gravitational force since he does both. And he forces his energy drain on people and he forces his gravity manipulation on his targets.
As for my character and Xavier... Idk, I'm TERRIBLE at judging how characters will mesh but I will say Marcus is quiet and jaded. He'll likely just expect Xavier to end up in a body bag some day so why get emotionally attach? Again, that might change. We'll just have to see because, again, I suck at this.