Leon Osvaldo is a French acrobat and a highly skilled trapeze performer who joins the Kaleido Stage in the second season of the story. He arrives after the departure of Layla Hamilton and is brought in by Kalos to revitalize the stage. From the start, Leon presents a cold, cynical, and demanding personality, far more so than Layla ever was, and he initially expresses disgust at the idea of performing with Sora Naegino, whom he deems unworthy. His ambition and obsession with perfection drive him to push his partners to extreme limits, a reputation that earns him the nickname "God of Death" because many of his previous partners ended up injured and forced to give up their careers.

Leon's background is rooted in tragedy. After the death of his parents, he and his younger sister Sophie fled abusive relatives and sought training under a trapeze coach named Alain. Together, they invented the Angelic Act, a complex and dangerous routine. However, Leon eventually left Alain, feeling he had nothing more to learn, and continued performing with Sophie. During a circus festival, Yuri Killian lured Sophie away with a false meeting request, causing her to rush back and be struck by a car in the street. She died in the hospital, with Leon beside her. Her dying wish was for Leon to find and raise the true star in her place. This loss crushed Leon and became the driving force behind his harsh methods: he subjects all potential partners to the same grueling training Sophie endured, seeking a successor capable of performing the Angelic Act.

Leon's role in the story is that of a rival and eventual partner to Sora. Initially, he refuses to work with her and instead chooses May Wong as his partner for performances such as "Dracula" and the Circus Festival, which they win with the "Demon Spiral." However, he becomes disillusioned with May's growing arrogance and drops her during an act, forcing her to reevaluate herself. As Leon observes Sora, he recognizes in her the same spirit and dream as Sophie—that of creating a stage where everyone can have fun without competition. This realization leads him to acknowledge Sora as the true partner he needs. In the final arc, he pairs with Sora for "The Swan Lake," helping her perform the Angelic Act. Through this collaboration, he not only fulfills Sophie's wish but also opens his heart to the idea of a joyful, conflict-free stage.

Leon's key relationships are defined by his past trauma. His bond with his sister Sophie is central; her memory colors his every interaction and motivates his relentless pursuit of perfection. His partnership with May is initially based on her skill but later soured by her pride. With Sora, he evolves from disdain to deep respect, finding in her the embodiment of Sophie's ideals. His antagonism with Yuri Killian stems from Yuri's role in Sophie's death, though Leon never directly confronts him. Over the course of the series, Leon undergoes significant development, moving from a closed-off, traumatized performer to someone willing to embrace teamwork and even comedic roles, as seen in the OVA "The Amazing Princess Without a Smile," where he plays one of the three Fools.

Notable abilities include his exceptional trapeze work, his mastery of the Angelic Act—a routine he and Sophie created from scrap materials—and his reputation as a "circus hero" and "stage Pegasus." His demanding training methods, while harsh, produce high-level performances, and his physical precision and discipline are unmatched. Despite his cold exterior, his journey demonstrates a capacity for growth and emotional release, culminating in a partnership that transcends competition.