He is suffering from nervous disorder, and has [[wikipedia:Parkinson's_disease|Parkinson's disease]. He will eventually get banished from Mesa Verde and appear in the slums of Hoover Dam, if the Prisoner makes him their puppet.
In Mesa Verde, he works in the greenhouse, but since he has Parkinson's, as well as Tourette's, his efficiency suffers. If the Prisoner cures him, this won't happen.
Radian has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.