David is Charles Xavier and Gabrielle Haller son. Seen in Episode "Sins of the Son" (voiced by Kyle Labine). In the comics David's personalities developed due to mental disorders (in the comics all his mental disorder personalities alternatively is also called Legion). David is disappointed in his father's absence while Lucas (one of his personalities) demonstrates this disappointment through anger and violence. David, considered the "true" personality, demonstrated no discernible powers.
Was altered slightly from the comics. He more closely resembled his father, (Professor X,) and had inherited his psychic powers, but here Legion was also able to shape-shift somehow to match whichever of his multiple personalities is dominant, with personality and body shifts sometimes happening at random. The mechanism behind this ability is never fully explained, although it is possible that David is using strong psychic abilities to alter people's perception of his appearance rather than actually changing as Mastermind had done when first trying to avoid being discovered. His personalities sometimes appeared in two places at once, supporting the control-of-perception theory. Only three personalities were shown (David, Lucas and Ian).
The dominant personality, He has psychic abilities superior to Jean's (formidable by this point in the series) and even caused Professor X's attempt to suppress him to backfire. Lucas lured Professor Xavier to Scotland and tricked him into locking David's other personalities away, leaving Lucas free to be himself. It was never explained what Lucas's goals were after this. The show was canceled before his storyline could be further explored.
Young, mute Ian possessed pyrokinesis, including the ability to create fire at will. His motivations remain unknown, but he sided with Lucas against David and the X-Men.
Multiple Personality Disorder
David is a fragmented, multiple-personality-afflicted individual. His personalities have fragmented into three, distinct personalities, that of himself, Ian, and Lucas.
David is a relatively docile personality.
Lucas is a violent, angry, manipulative personality bordering on sociopathic. He harbors hatred toward Charles Xavier, feeling abandoned and unloved, using his powers to get whatever he wants and hurt whoever he wants.
Ian is a manifestation of David's inner child, feeling neglected and abandoned. He is selectively mute and also harbors resentment towards Charles Xavier.
David's mutation grants him multiple, psionic abilities. David's mutation caused his dissasotiative identity disorder to actually manifest his two, alternate personalities, altering his body in a manner similar to shapeshifting. Each personality manifests a different aspect of David's mind and each possesses different, psionic powers. Lucas is the aggresive personality, manifesting incredible telepathic and telekinetic abilties rivaling the powers of Jean Grey and Proffesor Xavier. Ian is the passive personality, manifesting the power of pyrokinesis.
Ian is the most docile of the three, being mute and generally doing little. He appears to be a boy of about 9 and his loyalty goes with Lucas. His powers focus mainly on the pyrotechnic abilities. Lucas looks to be a Scottish male in his early to mid twenties. He is the most dangerous. Completely greedy, scheming and power mad, Lucas seems to be in control of the more considerable powers at David's disposal. His telepathy and telekinesis are much stronger than anything the X-Men have dealt with to date (except Apocalypse). Lucas conspired, using Ian as his accomplice and David as his pawn, to have his father meld them into one. He wanted to be free of their unpredictable changes from one personality to another and knew that Xavier was the only person powerful enough to rid him of the others. After Xavier's failed attempt to help David, Lucas gained control of their body and was last seen flying off into the horizon. But it is very likely he will be back, sooner or later.