Real Name: David Haller, although his alternate personalties go by the names of Ian and Lucas.
Mutant Powers: Telekinesis, the ability to read others' minds and project images into them, and the ability to psionically start fires (as Ian and Lucas); N/A (as David). It is unclear as to whether he actually PHYSICALLY changes when he switches personalities, or whether only his MENTAL PROJECTIONS into other people's minds changes.
Voice Actors: Kyle Labine and Ted Cole
Guest Appearance: "Sins of the Son"
Back Story: David is Charles Xavier's son, although the Prof didn't know about David until recently, when he had been "kidnapped" by another guy named Lucas. Xavier's ex-wife Gabrielle called him to Scotland, where they had recently moved, and it was soon found out that David was, in fact, suffering from a severe case of multiple personality disorder. Presumably, this was because of the emotional trauma David suffered as a result of growing up while thinking that his father abandoned him and his mother, or something. Anyways, David's alternate personality, Lucas, tricked Xavier into cutting off David and Ian's personalities from the rest of his mind, and now Lucas has full control over David's body.
Personality: David is certainly an enigma, that's for sure. Since his personality is split into three different people, I'll examine each of them in turn. David, first off, is the "real" personality out of the three, although it's not the most dominant one. It seems to be almost manipulated behind the scenes by the others, and only seems partially aware of the fact that he has multiple personalities. David is also the kindest of the three personalities, essentially the "good" side. David is also the only one of the three personalities who doesn't seem to possess any Mutant powers. Lucas, on the other hand, seems to be the most dominant personality out of the three (and certainly is now that Ian and David's personalites have been removed or locked away). Lucas is pretty much the polar opposite of David, and is the "bad side" of Legion. He's also quite the manipulator, and is completely unforgiving of Xavier's abandonment of him and his mother for all this time. Last but not least, Ian seems to be the "neutral" side of Legion, although it has sided with Lucas recently (which has probably been the key factor in making Lucas the dominant personality). Ian doesn't talk at all, and is actually a small boy, in comparison to the others, who are presumably in their late teens. Very odd, that. Ian also seems rather shy and distant.
Final Reflections: As much as I liked Legion, since he was such an original and intriguing character, he really should have been cut out of the series, since the problem at the end of the episode- Lucas' dominance of David- was never really touched upon again. That episode could've been used to more deeply examine other characters that had been around for a while-- like, say, Colossus or something.
(Pictures courtesy of Matt.)