Allen is an orphan who was abandoned by his biological parents because of the supposed "deformity" of his left arm (which was later revealed to be caused by his Innocence). He was eventually found and adopted by Mana Walker on Christmas Day. Though the circumstances were never revealed (though the anime supplies one possible cause with a scene of an overturned carriage), Mana died, and Allen, tormented with grief, turned Mana into an Akuma, hoping that his beloved surrogate father would be fully resurrected. But the Mana Akuma cursed Allen and sliced through his left eye, leaving the "Pentacle" -- the mark of the Akuma -- which scars the left side of his face and turns his hair white. This enabled his left eye to see the souls trapped within the Akuma from then on. It was at this same moment that Allen's left arm first invoked as an anti-Akuma weapon, which instinctively destroyed the Mana Akuma.

Allen eventually met Cross Marian, a general of the Black Order, who explained to Allen the ways of the Akuma and the trapped souls within them: Once a soul becomes trapped inside an Akuma, they are trapped forever; and that the only way to save one is to exorcise them from their Akuma vessel. Allen subsequently became Cross' disciple and spent the next three years with him before being sent to the Black Order to become an official exorcist.