Chiaki Nagoya, who operates under the alias of Kaito Sinbad, is the deuteragonist of the story and serves as both the rival and eventual love interest to the protagonist. By day, he is a new transfer student at Maron Kusakabe's school and her next-door neighbor, but by night, he transforms into the phantom thief Sinbad. To his classmates, he initially appears as the ideal popular boy: handsome, athletic, intelligent, and seemingly irresistible to many of the girls, including Maron's best friend, Miyako. This air of mystery is compounded by the fact that he lives alone, having left his family home, and he rarely speaks about his past. It is eventually revealed that his father is a hospital director who remarried multiple times following the death of Chiaki's mother, a situation that led Chiaki to run away from home.

His personality is defined by a significant duality that evolves throughout the narrative. When he first appears, Chiaki is characterized as a flirtatious playboy, often teasing Maron and acting in a way that she finds infuriating and despicable. However, this behavior is part of a calculated ruse. Having discovered that Maron is the reincarnation of Jeanne d'Arc and transforms into Kaito Jeanne, Chiaki, as Sinbad, moves next door and transfers to her school with a specific objective: to stop her from sealing away demons. He is aided in this mission by his own angelic companion, Access Time, who gives him the power to transform. At this stage, his approach is pragmatic and manipulative, aiming to seduce Maron to make her give up her duties as a magical thief. Despite this initial deception, his feelings for her develop into genuine affection, and he finds himself truly falling in love with her, which creates a deep internal conflict between his original mission and his authentic emotions.

As the story progresses, Chiaki's role shifts from that of a deceptive antagonist to a devoted protector. After Maron discovers his true identity as Sinbad and his initial betrayal, he dedicates himself to regaining her trust and fighting by her side. He is shown to be fiercely protective, though some interpretations of his character note that his protectiveness can manifest as jealousy or possessiveness. His motivations are driven by a desire to protect Maron from the unsettling truths of their world, particularly the revelation that her own angel, Finn Fish, is actually an agent of the devil using her. His backstory of feeling abandoned by his constantly remarrying father gives him a point of connection with Maron, who also lives alone due to her parents working overseas, and he often tries to cheer her up through small, thoughtful gestures, such as placing notes in her empty mailbox.

In terms of abilities, Chiaki possesses the power to transform into Kaito Sinbad, a rival thief whose powers mirror those of Jeanne. Like Jeanne, he has the ability to seal demons, typically converting them into black chess pieces rather than white ones, and he is equipped with his own magical tools to aid him in his heists. His appearance changes drastically during this transformation; while his civilian form has blue hair and brown eyes, Sinbad is characterized by whitish or silver hair and indigo or bluish eyes. As the series reaches its conclusion, Chiaki reconciles with his father and resolves his personal demons, eventually solidifying his relationship with Maron, which culminates in their marriage in the manga adaptation. There are deeper narrative layers to his existence, as later twists in the story suggest that Chiaki is the reincarnation of Adam, linking his fate with Maron (the reincarnation of both Joan of Arc and Eve) on a cosmic and biblical scale.