Chiaki Nagoya, also known by his alter ego Kaitou Sinbad, is a central character and the primary deuteragonist. He is a sixteen-year-old high school student who moves into the apartment next door to the protagonist, Maron Kusakabe, and becomes a new classmate in her year. He comes from a wealthy family, as his father is a doctor who owns a hospital. Chiakis childhood was marked by tragedy when his mother passed away at the age of five. Following her death, his father remarried and divorced multiple times, actions Chiaki viewed as an insult to his mothers memory. This led to a severely strained relationship with his father, and Chiaki eventually ran away from home, choosing to live independently in the apartment complex.

On the surface, Chiaki presents himself as a charming and handsome ladies man, a reputation that quickly makes him popular among the female students at school. However, his initial interactions with Maron are fraught with conflict; he often teases her, makes flirtatious and forward comments, and asks her to be his girlfriend, which she interprets as womanizing behavior. Behind this indifferent and cynical facade, Chiaki hides a genuinely kind and protective heart, shaped by his difficult family history and his initial disbelief in the value of love.

Chiakis primary role in the story is that of a rival to Marons alter ego, Kaitou Jeanne. He possesses the exact same abilities, transforming into the phantom thief Kaitou Sinbad with the help of his angel partner, Access Time. Unlike Jeanne, who is initially led to believe she works for God, Sinbad seals demons into black chess pieces. He moved next door and transferred to Marons school with a specific mission: to protect her. He was fully aware of her identity as Jeanne before they ever met, having learned the truth from Access Time, who revealed that Marons own angel, Fin Fish, had been deceived by the Devil. Chiaki adopted the persona of Sinbad to compete with Jeanne and prevent her from sealing demons, which would unknowingly weaken God and empower the Devil. His strategy involved getting close to and seducing Maron, believing that if she fell in love with him, she would stop being a phantom thief.

The relationship between Chiaki and Maron is the emotional core of the series. What begins as a calculated deception slowly transforms into genuine love. Despite his initial indifference, Chiaki finds himself truly falling for Maron as he witnesses her loneliness, strength, and kindness. He becomes fiercely protective of her, often risking his identity and safety to save Jeanne from danger. He shows his affection in subtle ways, such as leaving funny notes in her mailbox to cheer her up when she is sad about her absent parents. His development involves a profound internal conflict: he must reconcile his initial manipulative intentions with his authentic and growing love for Maron. This becomes critical when his secret identity is revealed, shattering Marons trust and causing a major rift in their relationship. He spends considerable time trying to prove that his feelings have become real and that his love is true. Ultimately, he overcomes this deception, and his devotion helps Maron in the final battle, leading to their future together.

In terms of abilities, as Kaitou Sinbad, Chiaki can sense and seal demons inhabiting works of art. He uses pins to perform the sealing ritual, though in the original manga adaptation, he uses daggers instead. He is a rational and quick-witted individual, often making calculated decisions. He is also a capable fighter and strategist, frequently arriving to save Jeanne from traps or direct attacks. His skills are a direct match for Jeannes, making him an effective rival and, eventually, a powerful ally.