Mana Walker on Christmas Day. Some time later Mana died, and Allen, tormented with grief and manipulated by the Millennium Earl, 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. 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. During the course of this traumatic event, Allen's hair turned from red (brown in the anime) into a stark white.
Allen eventually meets 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 is brought back inside an Akuma, they are trapped forever; 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. During this time, Allen was tormented by his master's constant irresponsible behavior and forced into various odd jobs to pay off the debts that General Cross accumulated. This, coupled with Cross' womanizing ways and harsh treatment towards Allen, led to Walker fearing more than respecting his master whom he never speaks fondly of. In fact, when Jasdevi claims that Cross is like a demon, Allen denies this and instead insists that Cross is a real demon. In addition, when Jasdevi reveal that the debts Cross left them reached 200 guineas, Allen grows demon horns, laughs at them, and calls their debts small change in comparison to his own, leaving Lavi to wonder just how much money Allen's been made to pay.
In addition to being physically fit and skilled at combat (as evident by his ability to fight and dodge Fō without the use of his Innocence), Allen has a few odd talents and skills. Because of the many debts Allen was forced into paying by Cross, he learned to play cards and gamble at a very young age, though in the anime his initial purpose was to practice his paralyzed and deformed left hand. He also learned how to cheat and win through deception and doesn't hesitate to use these tactics whenever necessary. In fact, he is quite proud of his talents. This is a stark contrast to his polite and courteous nature and often surprises his friends, who wouldn't expect such a dark side to Allen, and has caused them to wonder just what Cross has done to him. Walker also possesses circus abilities such as juggling and acrobatics, which he learned from traveling with Mana who was himself a circus clown by trade. Inspector Levrier and Cross have both referred to a "player" (as in one who plays a musical instrument), and Cross has hinted that there may be more than one.