Kai Shimada operates as a spy and officer in the Shinsengumi’s 2nd Division under Shinpachi Nagakura during peacetime. He sports a square jaw, dark brown hair, and a sturdy frame, typically clad in the Shinsengumi haori layered over a purple shirt and green trousers. As the group’s fortunes shift, his attire transitions to a black Western-style uniform adorned with gold buttons.

Debuting in the first season, Shimada infiltrates an inn undercover alongside Susumu Yamazaki, covertly gathering intelligence under his civilian guise. He engages in pivotal battles, defending Chizuru Yukimura against abduction attempts by Chikage Kazama. His combat prowess and loyalty shine during the Toba-Fushimi conflict, emerging as one of the sole survivors from his unit.

In the second season, following key departures, Shimada steps into a leading role, uncovering the secrets of Rasetsu transformations and executing critical missions under Toshizo Hijikata and Isami Kondou’s direct command. When Kondou stages a sacrificial diversion, he spearheads the retreat to safety. Later, he facilitates Hijikata’s escape from Utsunomiya Castle, detecting unsettling shifts in his commander’s behavior and cautioning Chizuru to stay watchful. His final documented engagement occurs during the Imperial Army’s Hakodate offensive, isolated from reinforcements with Ootori and fellow Shinsengumi fighters.

The third season, a prequel, explores Shimada’s origins within the Rōshigumi. Joining in the sixth episode, he identifies Shinpachi as a former dojo comrade. Tasked by Hijikata as an inspector, he partners with Yamazaki to probe internal affairs and external threats, maintaining operational efficiency despite limited screen time.

The character’s presence remains confined to the main series and prequel, with no documented appearances in films, OVAs, or spin-off media.