Chiaki Nagoya (also known as Kaitou Sinbad) is the deuteragonist of the anime and manga series, Phantom Thief Jeanne. He is partners with his angel, Acess Time and is also the main love interest of Maron Kusakabe.
Chiaki comes from a wealthy family as his father is a doctor and owns a hospital. However, when Chiaki was five years old his mother had sadly died. Ever since he has had a strained relationship with his father as he has constantly remarried and divorced several times over the years. Due to his father's constant remarriages, Chiaki never accepted love and believed it to be a waste. Later, Chiaki met Access Time, an angel that was sent to Earth to protect Maron as another Angel, Fin had been brainwashed by the Devil and also sent to Earth to trick Maron. Fin had deceived Maron, making her believe she was helping God, unaware she was really assisting the Devil all along. Access was exploring Earth when he and Chiaki first met and was surprised that Chiaki was able to see him, believing that Chiaki could help, Access explained the entire situation to Chiaki and he agreed to help.
Since then, Chiaki had been completely aware of Maron's identity as Jeanne and moved into Maron and Miyako's apartment building and began attending their school while also taking up the identity as Kaitou Sinbad, a new thief and becomes a rival to Kaitou Jeanne. His goal was to stop her from checkmating the demons otherwise God would grow more weaker while the Devil would grow more powerful. Chiaki also decided to seduce Maron, believing if he could make her fall in love with him, it would prevent her from continuing of being a Kaitou while stopping the Devil's plans. Maron was completely unaware of the situation and instead believed that Sinbad was a servant of the Devil and was distrustful of him, but on several occasions they've worked together and Sinbad has also saved her life multiple times, causing her to question his whether he is siding with the Devil or not.
When Chiaki and Maron first met, Chiaki was already secretly aware of Maron's identity as Kaitou Jeanne. Upon meeting Maron, Chiaki pretended to have feelings for Maron, believing if he could make her fall in love with him, she would stop being Jeanne. Chiaki never held any feelings for Maron when they first met and during their time together, he constantly teased and flirted with her as his first attempts while constantly asking if she would date him. Maron didn't like Chiaki when they first met as she believed him to be nothing more than a womaniser and a pervert. However, Maron slowly started to develop romantic feelings for Chiaki and eventually came to fall in love with him as he is the only person she has ever discussed the situation regarding her parents and began to realize that he truly was a kind person despite his attitude. Chiaki was also known to leave "funny" notes in her mailbox as a way of cheering her up upon learning the relationship between Maron and her parents.
When Chiaki was Sinbad, he declared himself to be Jeanne's rival as he always went to the same place where Jeanne was going to commit a theft and was always trying to prevent her from checkmating the possessed object and instead doing it himself. Maron as Jeanne had come to believe that Sinbad was a servant of the Devil and was collecting the demons to try and increase the Devil's powers, believing that was the reason she came to distrust him. Maron had also grown to to hate the way Sinbad was willing to do anything to seal the demons as some of his actions were completely unnecessary and ultimately quite cruel. Maron was also angered of his cold personality towards the victims that were possessed as he showed no sign of sympathy or regret for some of his actions. However, on numerous occasions, Sinbad had constantly saved Jeanne's life whenever she was almost caught by the police or if the situation was life threatening. During these times, it made Jeanne question if whether not Sinbad was truly working for the Devil, especially during moments when Sinbad showed signs of romantic feelings towards her, including when he kissed her twice. However, Maron came to believe this was another one of his plans of trying to prevent her from being a Kaitou and continued to distrust him.
Eventually, Chiaki had come to realize that he truly and really has fallen in love with Maron, but is still shy and conflicted with expressing his feelings as he felt differently about love, prior to meeting Maron. Since realizing his feelings towards Maron, Chiaki as Sinbad has shown to have taken protecting Maron more seriously, willing to either stand by her side or shield her from attacks. However, despite realizing his true love towards Maron, Chiaki still hadn't told Maron the truth regarding his secret identity as Sinbad or knowing her identity as Jeanne even before their first meeting. Chiaki was conflicted of telling Maron the truth regarding Fin, knowing she would greatly shocked and distraught upon realizing the truth and her powers would also be seriously weakened and felt it be better to not tell Maron.
Eventually, Chiaki finally revealed his feelings for Maron, assuring her she could trust him and even began wearing the special four leaf clover rings that Maron received to symbolise their love and to show Maron he was truly serious regarding his feelings for her. Maron was delighted to see Chiaki truly felt the same and they finally and officially became a couple and shared their first kiss. However, shortly after, during an attack against a demon, Chiaki as Sinbad was injured and the ring he wore was broken off and landed into Maron's hands as it was secretly a trap meant for both Chiaki and Maron. Maron as Jeanne had confronted Sinbad after the fight and had finally come to realize that Chiaki was Sinbad and was devastated to learn he had been lying to her right from the start. Maron was competely heartbroken of learning the truth regarding both her identity and of their relationship, believing Chiaki has never loved her and seeing he has only ever been manipulating and deceiving her feelings and emotions from the very beginning. Maron immediately broke off their relationship as she could no longer trust or believe him after everything he did, but is shown that despite it, she still deeply loves him.
Chiaki felt a great mount of guilt and remorse for what he had done to Maron and continuously apologized and told Maron how his feelings had changed and how he truly loves her. However, Maron kept avoiding Chiaki as she didn't want to be around him or even speak to him since learning the truth. Eventually, Chiaki decided to give Maron some space, knowing she wouldn't immediately forgive him and advised her to watch him closely to truly see if he was on the Devil's side. Eventually, Maron slowly started to forgive Chiaki, but their relationship was still shown to be somewhat tensed and strained since discovering the truth.
Eventually, once Maron learned the entire real situation regarding Fin and Chiaki, Maron and Chikai had worked together to save a brainwashed Fin. Later as they were attacking Fin's castle, Maron as Jeanne had left to confront her, but not before she gave Chiaki a kiss on the cheek, showing that Maron has finally and officially forgiven Chiaki while renewing their relationship. In the manga's ending, it is shown that Maron and Chiaki have gotten married several years later and have now become new parents to their newborn daughter, Natsuki, who is Fin's reincarnation.
Miyako is Maron's best friend who harbours romantic feelings for Chiaki.
Yashiro is Chiaki's arranged fiancée.
- Chiaki is known to have many female admirers including Miyako, Maron's best and childhood friend.
- Chiaki's mother died when he was child.
- Chiaki has a strained relationship with his father.
- Chiaki's Angel is Access Time.
- Chiaki decided to use the name Sinbad upon seeing the name on a movie board.
- In the manga and anime, Chiaki's identity as Sinbad was revealed at different times. In the manga, Maron discovered his identity in the beginning of the series, while in the anime, Maron didn't find out until the second season.