Sentarō Kotsubaki

is the co-3rd seat/acting lieutenant of the Thirteenth Division of the Gotei 13. He constantly argues with other co-third seat, Kiyone Kotetsu. Like Kiyone, he shows a great deal of admiration towards his assigned Captain, Jūshirō Ukitake.

Character Outline
Sentarō dresses a bit differently from standard Shinigami, wearing a white headband and white shoulder straps, both apparently made from the same material. However, 50 years before the main storyline, Sentarō and Kiyone both dressed closer to the standard Shinigami uniform.

Sentarō, like Kiyone Kotetsu, holds his captain in high regard and tries to impress him in any way possible. Both he and Kiyone usually get in fights over their competition to impress their captain. Oddly enough, they both share the same birthday as well, which further fuels their rivalry.

History
Not much is known about Sentarō's history, except that he came from Rukongai. He was already a member of the 13th Division by the time Rukia Kuchiki joined. Both he and Kiyone would occasionally show up on duty drunk, as noted by Lieutenant Kaien Shiba (who was frequently annoyed when they did so). It is unknown what rank Sentarō had when both Kaien and Miyako were still alive, but he (and Kiyone) became a 3rd Seat officer sometime before the main storyline.

Soul Society arc
Sentarō first appeared alongside his partner Kiyone Kotetsu when they inform Captain Jūshirō Ukitake about Sōsuke Aizen's apparent death. Both Sentarō and Kiyone decide to take the blame for delaying the message to Ukitake, since he was sick. Later, both he and Kiyone follow Ukitake when he stops Byakuya Kuchiki from killing Hanatarō Yamada and Ganju Shiba. They do not show themselves until after Byakuya leaves. Ukitake orders Sentarō to take Rukia Kuchiki back to her cell, while Kiyone gets a Fourth Division medic to treat Ganju's wounds. When Hanatarō tries to stop him, Sentarō admits that he does not want to put her back in her cell, but states that he has to follow orders.

Later, Sentarō and Kiyone bring Hanatarō to Retsu Unohana, and Sentarō gives her a report that states that Hanatarō was forced to help the Ryoka, but Unohana still punishes him, despite this.

When Ukitake's appeals to Central 46 fails, and the execution date is moved, he decides to save Rukia himself, and Sentarō and Kiyone decide to assist him. They arrive at Sōkyoku Hill, and Ukitake, along with Shunsui Kyōraku, destroys the Sōkyoku. After Renji Abarai escapes with Rukia, and after Ichigo Kurosaki begins battling Byakuya, Kiyone tries to help her sister, and Sentarō tries to stop her, but he is knocked unconscious by Soifon, who also begins to kill Kiyone after Shigekuni Yamamoto-Genryūsai leaves to follow a fleeing Ukitake and Kyōraku (who stated it was better to fight elsewhere, so the others wouldn't get caught in the crossfire). They are ultimately saved by Yoruichi Shihōin, who drags Soifon away to do battle elsewhere.

After the danger around them is passed, Unohana takes Sentarō and everyone else wounded at Sōkyoku Hill (except for those who are still fighting) to the 4th Division to be treated. Later, while Sentarō is in recovery, he receives Isane Kotetsu's message elaborating on Aizen's betrayal.

Zanpakuto

 * Shikai has yet to be revealed.