The child of mutant telepath Charles Xavier and geneticist Moira MacTaggert, David Xavier grew up believing his father didn't love him. Certainly the intellectual Charles found it difficult to relate to his son, who loved soccer, video games, and wrestling. When Charles left his family to found the Brotherhood of Mutants with Magneto, he firmly believed they wouldn't miss him. But David did, and the very next day his own mutant powers disastrously emerged. Their slightest use ravaged David's body, so Moira was forced to keep him sedated; he spent several years in his mother's care in the Muir Island hospital. Somehow sensing his father's return to Britain, David suffered a fatal convulsion but survived by transferring his consciousness into nurse Isobel MacLinden. He fled to the mainland, swapping bodies as each host wore out and slaying anyone who got in his way. Confronted by his parents and the X-Men in Aberdeen, he usurped Wolverine's form and fought them with a combination of reality manipulation, telepathic psychological warfare, and his host's natural skills. Since Wolverine's regenerative powers countered the internal damage David's powers caused, David intended to retain his new host permanently; but while attempting to flee, he carelessly leapt in front of an oncoming truck. David jumped from Wolverine in to the driver seconds before his former host was run over. After the vehicle crashed, David secretly possessed telepathic S.T.R.I.K.E. agent Betsy Braddock. Having finally figured out how to slow down his degeneration, he hid within Betsy and lured the X-Men to Berlin, where the Xaviers had held their last family holiday prior to Charles leaving.
As Betsy, he quizzed his father to learn Charles' true feelings for David, unaware that Charles knew his true identity. When Charles revealed this, David abducted him. With his powers growing, David restructured Betsy's body to resemble his original one, overpowered the X-Men and critically injured Iceman, then teleported Charles and himself around the world, blowing up landmarks and murdering innocent people. He taunted his father, claiming Charles didn't care about him,, and sarcastically suggesting he might join the X-Men as Proteus. As he prepared to kill Charles, Betsy's mind resurfaced, blocking David's powers. She urged Charles to kill them before David could resume control or escape, but Charles found himself unable to kill his own son or violate his principle of non-violence. The X-Man Colossus had no such problem, and swiftly crushed Betsy/David beneath a car, apparently killing them both. However, Betsy was reborn in a new body, so David's final fate remains in doubt.