Leon is a French acrobat with a hard and cold character. He is recruited by Kalos to Kaleido Stage but his disgust of Sora Naegino almost makes him terminate his own contract very early on. However, after seeing Sora and May Wong fight on stage, he decides to remain in hopes that one of them can become his true partner. According to Layla, he's so hard on his training and treatment of all partners that almost all of them end up injured and have to give up, earning him an horrible reputation among artists as well as the nickname "God of Death".

Leon first chooses May, who becomes his partner in the work "Dracula" and again in the Circus Festival (which they win after performing the "Demon Spiral"). However, he realizes that the partner he truly wants and needs is Sora Naegino, who reminds him of his little sister Sophie, particularly in her dream: "creating a stage with no competition where everyone will have fun." May had also become arrogant and self-centered, so he drops and injures her during an act, forcing May to re-evaluate her worth as a performer.

The introduction of his little sister Sophie reveals Leon's stormy past. After their parents died, Leon and Sophie abandoned their abusive relatives and sought a trapeze coach named Alain, who took the siblings under his wing and discovered their potential, as well as how they invented put of scrap the Angelic Act. He subjected both of them to an amazingly hard training (especially Sophie, who was the one with the strongest will to perform the act), but at mid-way Leon felt that Alain couldn't teach them anything and left with Sophie.

During the last Circus Festival, Sophie was called by Yuri Killian asking her to meet him before the Festival at which she and Leon were two perform. However, this was a trap: Yuri never meant to meet with her, only wanting Sophie out of the way to cause the pair's forfeit and solidify his win with Layla. Just before the Festival began, however, Sophie tried to come back in time, only to be hit by a car when running across the street; she died in the hospital, with a crushed Leon next to her. Just before she died, Leon called Yuri and tearfully asked him to visit Sophie, trying to deny the rumors about him being the culprit; Yuri, however, knew that he couldn't just show up and didn't.

Sophie's death crushed Leon as he struggled with trying to fulfill his sister's dying wish: find and raise the true star in Sophie's place. For that, he subjects all of Sophie's "successors" (including May and Sora) to the same training Sophie went through, which is the cause of his "God of Death" nickname. He eventually realizes, when he meets Sora, that she is the one who can become his partner, perform the Angel Act and become the true star.

In the final chapter, Leon is paired up with Sora in the work "The Swan Lake", and after helping Sora perform the act with her, not only fulfills Sophie's wish, but also opens his heart to the idea of performing on a fun, conflict-free stage just like the one his sister dreamed of.

In the OVA The Amazing Princess Without a Smile, he tries to take up more comical roles on the Stage, playing one of the three Fools.