In my mind, Orin is the most dysfunctional Incandenza son. He has no contact with his family, other than cryptic calls to Hal. He's addicted to one night stands with women he can't wait to get rid of afterwards. He has a phobia about roaches and deals with it in an overly obsessive way (tumblers, Ziplocs). He fears the dawn, whereas other characters think of the "sadness" of the day becoming night.
Mario, despite his physical limitations, seems to be the most open and well-adjusted son emotionally. People feel comfortable talking to him and sharing their feelings. Mario doesn't feel self-conscious, he waves his "claw" at folks as he zooms down Comm. Ave. on the motorcycle with Schtitt. Mario isn't afraid to discuss painful issues, as in his conversation with Hal about whether the Moms grieved after Himself's suicide.
Hal, at this moment, could go either way. He's outwardly acting like Orin, with his addictive pot smoking, and moving up in the ranks over the past year (increased focus on achievement), as well as his "acting cool" around his Little Buddies. But he still questions the "rightness" of his own actions, even if he attributes it to "marijuana thinking", so he hasn't become totally cynical yet.
Hopefully no spoilers above. It's tough when you're reading both the beginning and end of the book simultaneously.
