Xavier is a rather quiet and reserved guy. He doesn't appear to get out much, and when he does go out, he doesn't like to talk with anyone unless he absolutely has to. Though, since he tends to almost always look bored or annoyed, not talking to people isn't that hard as he doesn't seem approachable. Wile he does appear to not get out much, he actually spends most of his time away from his home. He works with a well respected scientist and spends most of his time at the lab, but his employment there, as well as what he does, is kept strictly on a need to know basis.
Though he always seems uninterested in whatever is going on, Xavier is actually extremely observant and notices the subtlest of changes in others behavior. He can read lips, so even when he's at a distance he can pick up on any conversation happening around him. His doesn't put his observation skills to any professional use, though, and prefers to just make mental notes about what he sees and hears, in case any of that information becomes useful at a later date.
Xavier tends to be on the laid back side around those he knows, but he does have a bit of an aggressive streak. When he feels threatened or irritated, its not uncommon for him to lash out with threats, and he's been known to fight when his threats aren't taken seriously. He's generally not afraid of anyone and has a low pain tolerance, so winning fights comes easily and naturally to him.