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 Millennium Earl split into two and thus Mana and Nea was born. As the Noah blood with in them start to surface, they both knew that one of them will have to devour the other in order for them to become the Millennium Earl again. Mana, the older brother, decided to consume his younger brother Nea. After Mana has killed and consume Nea, his guilt for killing his own brother caused him to believe that he is not Mana but the Millennium Earl. That Mana and the Millennium Earl are two different people.