Mana Walker is Allen's foster father. Allen was originally abandoned by his biological parents because of his deformed left hand, which was due to his Innocence. When Mana found Allen, he decided to adopt him. Over time, Allen formed a very strong bond with Mana. It is unclear how Mana died though in the anime the secene of an over turned coach was shown. For Allen, Mana was like his father so he mourned for him. The Millennium Earl came to a grieving Allen after Mana died and offered to bring him back to life. Of course, as Allen had loved him, he accepted. Mana was then turned into an Akuma and infuriated at Allen's actions, cursed Allen by cutting the boy's left eye leaving a pentacle marking and also giving him the ability to see Akuma with his left eye. He was quickly destroyed by Allen's awakened anti-Akuma weapon. Before his Akuma form got destroyed, he told Allen that he loved him, and thanked Allen for righting the wrong he had committed.

A born exorcist, Walker's primary anti-akuma weapon is the cross that's embossed on his red, disfigured left hand, which contains Innocence. But not only does Walker destroy akuma, he sees the akuma hiding inside a person's soul! Together with his fellow exorcists fighting under the command of the Black Order, Walker leads the battle against the Millennium Earl, the evil being out to destroy mankind.
 
(Source: Viz Media)

Towards the end of the 19th century, Allen Walker officially joins the organization of Exorcists that destroy the beings known as Akuma; mechanic weapons made by the Millennium Earl with the suffering souls of the dead. Allen has both a cursed eye and an anti-Akuma weapon as an arm, bearing the power of "Innocence", a gift given to him as an apostle of God. Allen, along with his fellow Exorcists must put a stop to the Millennium Earl's ultimate plot that could lead to the destruction of the world and all who live on it.

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Worked for ADV Films and currently Funimation. He's an ADR Director, Recording engineer, and voice actor for numerous works.

Despite the recent Akuma attack, the members of the Black Order are in high spirits as they set about moving to a new base. Immediately upon his arrival, however, Allen Walker is suddenly called by the Central Agency and has his arm forcefully sealed by the Order. He is then led to a surprising meeting with his master, General Cross Marian, who reveals staggering secrets surrounding the enigmatic 14th Noah. 

A phantom thief incident, the arrival of a mysterious group of Exorcists, the death of an important comrade, and an all-out battle against the Noah Family—just what does this mean for Allen, Yuu Kanda, and the rest of the Order? This is but the beginning of a series of strange, seemingly unconnected events that lead to something far greater. 

(Source: MAL Rewrite)