Allen is the main hero of the series. He 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 a traveling performer named 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 brown 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 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. 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 Marian's 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 to them total about 2 million yen, 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 Fou 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 his initial purpose was to practice his numb 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.
Allen's synchronization ratio reached a maximum of 83% before having his Innocence destroyed by Tyki. Yet unknown to Allen, the Innocence lingered around as a hazy fog of particles; at the time, a direct reflection of his shattered state of mind. It wasn't until he came to an epiphany that he was able to seize control of his Anti-Akuma weapon once again and achieving his Innocence's true, fully-invoked form. When not activated, his arm is black and meets with his shoulder in a spiraling pattern that ends with tattoo-like emblem. Following this, the Order gave it the official designation of Crowned Clown (神ノ道化（クラウン・クラウン）, Kuraun Kuraun), due to its jester-like appearance when invoked. The kanji used in the name literally translates to God's Clown (神ノ道化, Kami no Douke).
When invoked, Crowned Clown acts as a body-armored Innocence, forming a white-hooded cowl and sleeve of Innocence energy which covers his shoulders and right arm, while his left arm and hand become slender and black, clad in armor and razor sharp claws. A masquerade-style mask is always affixed to the cowl, which takes on various configurations and appearances. This transformation also changes Allen's usual hairstyle from flat to a spiky rear. As before, his Innocence possesses a certain degree of sentience, but as Crowned Crown is able to manipulate Allen's entire body like a marionette when incapacitated, or simply assisting in evasive tactics.
When facing Tyki yet again, Allen synchronization with his Innocence exceeded 100% which activated his new Crowned Crown ability, Sword of Exorcism. The Sword of Exorcism has the ability to exorcist an Akuma on touch and even the ability to exorcist the Noah Gene from Noah's themselves. What shocked Tyki the most wasn't the ability of the sword but rather the similarity between Allen's Sword of Exorcism and the Millennium Earl's sword.
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.