AboutAllen is initially rather short, though in the manga he seems to grow as it progresses. He has white hair (a side-effect the curse Mana Walker inflicted upon him) that grows longer and spikier over the series, as well as a red (sometimes seen as purple) scar along his left eye, an injury inflicted by Mana that resulted in his left eye being able to detect the souls of Akuma. Because of his Parasitic Innocence, his left arm is red and somewhat unnatural. Allen's Innocence's form was destroyed in China, though he regains it shortly thereafter and it has since taken a black, more human appearance when deactivated, the skin smoother and no longer plated. Allen wears the standard Exorcist uniform, with a hood and gloves covering his hands. Allen is somewhat slender, but physically fit, with muscle tones not normally seen on boys of his age. He has a rather prominent scar along the right side of his torso caused by an encounter with a level four Akuma. After his training to reclaim his Innocence in China, Allen was given a communicator earring, which continues to be a new part of his wardrobe while it has received modifications several times.
Allen Walker is a very kind and generous person. He is nice, caring, and well-mannered, always polite. He is quite close to his friends, namely Lenalee Lee and Lavi. Always thinking of the lives of others before himself, he often puts himself in harm's way for other people's sakes. After his family had abandoned him because of his strange left arm, he lived at a circus where he performed odd jobs for money. His personality was different in his childhood. He was rude and cold towards Mana, Cosimo, and the other circus members. He would hit anyone who provoked him, and cursed anyone who bothered him. Allen, Mana's dog, was the only animal that cared for him and the only animal that he cared for. After Allen's death, Mana and Allen (who was called Red in the circus) became friends which formed from the bond of the beloved dog. Because of his experience with Mana as well as being cursed from turning Mana into an Akuma, Allen considers Akuma to be extremely important to him. He wishes to soothe the suffering of the Akuma's human soul by freeing it with his Innocence. He always remembers his promise to Mana, to always continue to walk, to move forward.
Allen is seen as a true gentleman throughout his ventures, but he hides the most strife and anger seen through any of the characters in the series. When observing Allen's ventures, he has caused the death and prolonged passing of his adoptive father, Mana Walker, has lost his master, with whom he shared a deep relationship with, was abandoned at birth due to his deformed left arm, and has failed to protect the various characters in the series that he has formed deep bonds with, such as Tapp, whom he felt responsible for his death after the Skulls transformation. Allen has lost those he has truly formed deep bonds with, has pushed himself into an emotional corner with self-blame, and feels like an outcast from the very humans he protects. Even with all of these negative situations constantly being pushed into his face, he continues to use the Innocence that has so drastically ruined his life to protect the world from the Noah and the Akuma.