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.
It is revealed in the manga in chapter 219 that Mana is half of the Millennium Earl. Somehow, the original Millennium Earl gave up on his mission and split into two and thus Mana and Nea was born. The two were raised by Katerine Eve Campbell, the former Earl's lover. When their Noah awakening came about Mana was embraced as the new Earl whereas Nea was rejected as an "extra" Noah not meant to exist. The two rejected their heritage, killing the other Noah, and went on the run, but eventually Mana's noah memory usurped him to kill Nea in vengeance. Mana regained consciousness 30 years later, with no memory of killing Nea or awakening as the Earl. He begun travelling as a clown to hide himself from the Earl's golem suit, which was hunting him, and find his missing brother. He found Allen one Christmas eve while taking on a new contact and took him in as his assistant after Mana's partner, a dog also named Allen, was murdered. It was actually Allen's decision to start traveling with Mana as his caretaker, after Allen accidentally broke Mana's mind when his innocence awakened, rendering him demented.
