Kotaro Shirai is a young civilian who acts as a close friend and supporter of the Kamen Riders in the alternate timeline of the film Missing Ace. Four years after the main series' events, he has achieved significant personal success, having published a best-selling book based on the adventures he witnessed alongside Kenzaki and the others, which has made him wealthy. He is the younger brother of Haruka Kurihara and the uncle of Amane Kurihara. As a character, Kotaro is earnest, grounded, and optimistic, providing a stable human perspective amid the superpowered conflicts. Because he has no combat abilities, his role highlights the stakes for ordinary people caught in the battle against the Undead. His motivations are driven by a deep loyalty to his friends and a strong protective instinct toward his family, particularly his niece. While he starts the film enjoying a successful civilian life, his involvement is renewed when the Albino Joker targets his niece Amane to use her as a sacrifice. This personal threat pulls him back into the conflict alongside the original Riders. Kotaro shares a strong bond of friendship with Kazuma Kenzaki and remains close to former BOARD operative Shiori Hirose. His relationship with Amane is the emotional core of his role in the story, as her safety becomes a central concern. He also interacts with the returning Riders Hajime Aikawa and Mutsuki Kamijo as they reunite to stop the Albino Joker. Kotaro's development in the film shows his maturity from an aspiring writer chasing a rumor into a successful author who has preserved the Riders' legacy. His arc sees him return to the chaos of the Rider conflict from a life of peace, where his primary contribution is not physical strength but his unwavering solidarity, support, and the moral grounding he provides. His notable ability is his writing, through which he memorialized the Riders' exploits for the public.