Mitsuru Ichihoshi is Hikaru Ichihoshi's older brother, two years his senior. He has dark blue neck-length hair, sea-blue eyes, and a fair complexion, typically wearing a yellow short-sleeved jumper with white stripes, blue shorts, and white shoes featuring blue stripes.

Cheerful and enthusiastic, Mitsuru possesses a deep passion for soccer. He maintains a close, caring bond with Hikaru, frequently seeking his younger brother's advice to improve his soccer skills. The brothers often practice together, united in their aspiration to follow their father's path and compete internationally in soccer, a goal their father encouraged while emphasizing its challenges, which Mitsuru met with determination.

A car accident tragically claims their father's life. Mitsuru saves Hikaru from the wreckage but sustains fatal injuries himself. This loss leaves Hikaru physically injured and psychologically traumatized. Overwhelmed by grief and guilt, Hikaru develops Dissociative Identity Disorder (DID), unconsciously adopting Mitsuru's identity as a coping mechanism. This splits Hikaru's psyche into two distinct personas: his original self and one fully embodying Mitsuru. When the Mitsuru persona emerges, Hikaru's hair slightly puffs outward and his pupils display a bright blue ring. This persona typically manifests outdoors, while Hikaru's original identity surfaces during his confinement in a hospital wheelchair.

Following the accident, the Orion Foundation, an organization with hidden agendas in global soccer, takes custody of the brothers. They manipulate Hikaru by fabricating a serious illness requiring specialized surgery in America, coercing his cooperation with the promise of treatment. Under this manipulation, the Mitsuru persona becomes an operative for Orion, infiltrating the Inazuma Japan national team as a spy. During missions, this persona engages in sabotage: attempting to injure players like Endou Mamoru with modified equipment, framing teammate Kidou Yuuto for doping, and collaborating with opposing teams to undermine Inazuma Japan's matches. These actions are driven by Orion's leverage over Hikaru's perceived medical condition, creating internal conflict as the persona wrestles with using soccer for harm.

A turning point occurs during Inazuma Japan's match against the Arab no Hinotori Gundan. Confronted by teammates and pressured by Orion's escalating demands, Hikaru confronts the suppressed truth of Mitsuru's death. This triggers a psychological integration of the two identities. Hikaru acknowledges both his own existence and Mitsuru's legacy. The merged identity retains Hikaru's core self while preserving Mitsuru's memory as an enduring inspirational force. Post-integration, Hikaru adopts a polite demeanor and demonstrates heightened analytical abilities in soccer strategy. He actively supports his teammates to atone for prior actions, expressing deep remorse for the Mitsuru persona's deeds and fearing he jeopardized team cohesion, yet remains committed to protecting his newfound friendships. Mitsuru's influence endures symbolically; Hikaru frequently reflects on their shared motto of being "the strongest team" and draws strength from their united spirit.

In this integrated identity, Hikaru honors Mitsuru's legacy through his continued soccer career, employing his tactical skills for positive contributions. He carries Mitsuru's memory forward, emphasizing themes of resilience and eternal familial bonds.