Koei, also known as Ninja Rider Koei, begins as an antagonist within Gabu Samejima's Team Shark Tooth. His actions are driven by a desperate need for funds to cover medical treatment for his younger sister, Kiku, who suffers from an illness; this desperation previously led him to steal before joining the team.

He possesses long, grey frizzled hair tied at the back and wears a green outfit with numerous pockets holding blades and ninja weapons. His Idaten Bike, Aero Scissors, enables his signature "Aeroblade Tornado," unleashing a hurricane around him. Koei also wields ninja techniques like medicine-infused smoke bombs, mini-mirrors for illusions, and the ability to ride on thin wires.

Despite his fierce role in Team Shark Tooth, Koei exhibits a kind-hearted and protective nature, especially towards younger children. He forms a close bond with Ayumu Yamato, the youngest member of Sho Yamato's group, later mentoring him in bike control. His loyalty shifts dramatically when Gabu kidnaps Kiku to coerce him into winning the Idaten Battle Tournament. Defying Gabu, Koei allies with Sho Yamato to rescue Kiku. He then leaves Gabu's employ and aids Sho's group in reclaiming stolen Gold Emblems.

After departing Team Shark Tooth, Koei assists Sho's group in defeating them and subsequently confronts Team X. During this conflict, he is briefly controlled by the Dark Emblem but freed by Sho. Koei delegates responsibility for protecting X-City to Taiga Samejima while remaining with Sho's group, continuing to support them in battles against Team X, including the final clash against Takeshi Yamato.

Koei's development transitions him from a morally compromised antagonist to a trusted ally, highlighting his dedication to family and redemption. His relationships, particularly his unwavering commitment to Kiku and mentorship of Ayumu, remain central to his narrative.