Who is Allen Walker?

Answer 1

In the anime series D.gray-man, Allen Walker, a young Exorcist who joined the Black Order after training with his master General Cross Marian. When presenting himself to the organization, he is mistaken for an enemy and is attacked by another Exorcist, Yu Kanda. However, once the mistake is realized, Lenalee Lee prevents any Kanda from causing further mishap. Once inside the headquarters, Allen learns he is the prophesied "Destroyer of Time" who is the only one who can defeat the Millennium Earl. As the series progresses, Allen meets many of the members of the Black Order headquarters as well as those in other branches worldwide. He eventually meets a young, aspiring bookman called Lavi and his predecessor Bookman, who follow Allen on his adventures to witness and record the events for the bookman's record. Allen also encounters the antagonist the Millennium Earl, his akuma army, and the Noah Family, a group of humans who aid the Earl in his plot to destroy humanity.

Answer 2

He is the fictional protagonist of D.gray Man (manga 2005 and anime 2006).

No one knows exactly how old Allen is because he was abandoned by his parents. He is thought be 15 at the start of the series.

The whole order also claim that he's 15 as well. But after he gets his arm back (2nd-3rd series), he's 16.