In his human form, Leo is a young boy with fair hair and gray eyes. He wears a blue jacket over a black shirt and light colored pants.
In his Akuma form, Leo takes the form of a typical Level 1 Akuma.
Leo is only seen talking a handful of times, but, like Jan, he did a lot of research about Akuma and seemed curious, though he displayed a much more visible level of disgust and jitteriness than Jan upon learning how Akuma are made. After the death of his mother, Leo cut himself off from Jan and fell into a deep enough depression to summon the Earl and bring his mother back as an Akuma.
Leo developed a curiosity for Akuma when he learned about them from his friend, Jan, whose father did work for the Black Order. He would often accompany Jan when he went on patrols through the town, looking for Akuma. When his mother died, though, Leo fell into a deep depression. It worsened to the point that he used his knowledge of the Earl to sacrifice himself to bring her back, despite the knowing the consequences of such a contract, resulting in his soul being lost and his body becoming a Level 1 Akuma. Jan never suspected a thing.
Leo is first seen visiting Jan after Jan returns home upon assaulting and escaping Allen using an "onion bomb." Though he doesn't say anything, Leo listens to Jan talk about an Akuma he had recently seen, and when Jan admits that he saw the Earl's face, a strange symbol forms in Leo's eye. After Jan draws a picture of the Earl for him, Leo takes Jan's hand and tells him he wants to show him something.
Just as Allen Walker arrives to continue trying to dissuade Jan from looking further into Akuma and the Earl, Jan and Leo are preparing to leave the house. Jan pushes one of his maids over onto Allen to stall him, and as he and Leo make a run for it, Allen's eye reacts to something about Leo. Before Allen can warn Jan, he and Leo skate away.
Leo leads Jan to the town cemetery, where Jan, thinking Leo wants to pay his respects to his mother, enters, only to run right into the Earl. When Jan tries to run away with Leo, Leo refuses to move, and the Earl reveals that "Leo" has actually been an Akuma for a while. Jan doesn't believe him, but Allen shows up with his Innocence activated and confirms it.
When Jan tries to talk to Leo, Leo just smiles at him silently, and when Allen tries to attack Leo and destroy him, Jan stands in between them and shields Leo, demanding that Allen proves that Leo is an Akuma. In response, Leo takes on his Akuma form and aims his cannons at the back of Jan's head. Leo fires, and Allen shields Jan and is quickly poisoned by the Akuma Virus.
The Earl steps in and expresses his annoyance with Jan, explaining that Leo's form is a manifestation of the "yearning in [his] heart", and that Akuma are mad with "everone's best interests at heart".
Allen then explains that Leo's Akuma was made using Leo's deceased mother's soul, and that he can see her suffering, the Earl becoming interested and infuriated by the idea of Allen being able to see Akuma souls. When Allen recovers and suppresses the Akuma Virus within himself, purifying his body with his Innocence, the Earl begins to ride Leo and has him open fire on Allen. When Allen fends all of the Akuma Blood Bullets off, the Earl summons more Akuma to fight.
Jan, realizing Leo's decision to become an Akuma was his fault, shouts out for Allen to destroy Leo as Allen attacks, and Allen uses his Cross Grave to destroy Leo and the other Akuma.
Later, after Jan has taken Allen back to his house to recover, Jan is seen soldering a cross to act as a gravemarker for Leo, explaining when Allen asks that Leo is being treated as a runaway and won't have a grave for sometime yet. The cross, though, will be Jan's replacement until the town decides to bury Leo. After expressing his wish to Allen to join his father in his work, Jan puts the cross on a shelf next to a picture of him and Leo.
Human Form: As an Akuma, Leo is able to disguise himself as a human and go about unnoticed.
Blood Bullet: As a Level 1, Leo has access to only the most basic of Akuma abilities, the Blood Bullet.
- D.Gray-man Manga Volume 1, Chapter 4, Page 119
- D.Gray-man Manga Volume 1, Chapter 4, Page 120
- D.Gray-man Manga Volume 1, Chapter 2, Pages 76-77
- D.Gray-man Manga Volume 1, Chapter 2, Page 79
- D.Gray-man Manga Volume 1, Chapter 2, Page 81
- D.Gray-man Manga Volume 1, Chapter 2, Page 82
- D.Gray-man Manga Volume 1, Chapter 2, Page 85
- D.Gray-man Manga Volume 1, Chapter 2, Page 87
- D.Gray-man Manga Volume 1, Chapter 3, Page 95
- D.Gray-man Manga Volume 1, Chapter 3, Page 99
- D.Gray-man Manga Volume 1, Chapter 4, Page 117
- D.Gray-man Manga Volume 1, Chapter 4, Page 122
- D.Gray-man Manga Volume 1, Chapter 4, Page 131
- D.Gray-man Manga Volume 1, Chapter 3, Pages 94-95
- D.Gray-man Manga Volume 1, Chapter 4, Page 115