Chiaki Nagoya is a central figure in the story, leading a double life that places him as both a neighbor and a rival to the protagonist. His outward identity is that of a handsome, popular, and somewhat flirtatious high school student, but he secretly operates as the phantom thief Kaitou Sinbad.

Physically, Chiaki is described as tall and lean, with blue hair and brown eyes. He is athletic and has a clean, casual fashion sense when not in his school uniform. His family background is marked by loss and conflict. His mother passed away when he was five years old. Unable to accept his father repeatedly remarrying after her death, Chiaki ran away from home and began living alone in an apartment building, which coincidentally places him next door to Maron Kusakabe.

On the surface, Chiaki comes across as a playboy who enjoys teasing others, particularly Maron, with whom he has a contentious relationship from their first meeting. However, this behavior is a mask for a more guarded and rational personality. Unlike the emotionally driven Maron, Chiaki is initially depicted as someone who claims not to believe in love and who makes decisions based on logic and his mission. Despite this cynical facade, he possesses a core of kindness that stems from his own emotional wounds, and he is often the first to act protectively toward those he cares about.

Chiaki's primary role in the story is that of a rival phantom thief. As Kaitou Sinbad, he appears to work against Jeanne, competing with her to seal demons and seemingly serving the devil. He is aware of Maron's secret identity from the start, while she remains ignorant of his for a significant period. His true motivation, however, is more complex. He initially approaches Maron and becomes a rival to stop her from her mission, not out of malice, but because he knows that Finn, Maron's angel companion, is actually an agent of the Devil. Chiaki's goal is to prevent Maron from being used and ultimately destroyed.

Chiaki is partnered with a small angel named Access Time, who has dark wings and who grants him the power to transform into Sinbad. Unlike the tense relationship between Maron and Finn, Chiaki and Access are described as good friends. As Sinbad, Chiaki uses black pins to seal demons, contrasting with Jeanne's white pins. He is often more ruthless and direct in his methods, willing to go to greater lengths than Jeanne to achieve his objective.

The most significant relationship in Chiaki's life is with Maron Kusakabe. He moves next door and transfers to her school specifically to get close to her. What begins as a calculated strategy to seduce her into giving up being a phantom thief gradually transforms into genuine affection. Despite his initial intentions, he finds himself blushing at her smiles and truly wanting to protect her. He demonstrates his care in small but meaningful ways, such as leaving notes in her mailbox after noticing her sadness at receiving no mail from her absent parents. As both Chiaki and Sinbad, he repeatedly saves her from physical danger and supports her when she is emotionally vulnerable, eventually becoming her love interest. By the end of the story, this relationship culminates in them becoming a couple.

Chiaki undergoes a significant evolution from a cynical and manipulative rival to a devoted and loving partner. Plagued by his family issues and his singular mission to stop Maron, he initially denies the feelings he develops for her. Her trust is severely damaged when she discovers his true identity and his original scheme to seduce her, forcing him to confront his genuine emotions and fight to earn back her faith in him. He also reconciles with his father later in the series, introducing him to Maron and beginning to heal the rift caused by his mother's death. His journey is one of learning to believe in love and emotional connection, moving past his initial pragmatic worldview.