Iko Koishikawa is a teenage girl who becomes a central figure in the life of the Japanese hero Shy. She is a student of the same age as the protagonist, Teru Momijiyama, and her path crosses with Teru's during a major incident at an amusement park called Himawari Land. While on a roller coaster with her younger brother, the ride malfunctions, and Shy arrives to rescue the passengers. In a selfless act, Iko urges the hero to save others before herself, which results in Iko suffering a critical injury to her right leg and losing consciousness. This event deeply traumatizes Teru, but Iko survives. Due to her injury, she transfers to a school closer to her home, which happens to be the same one Teru attends, and the two become classmates.

In personality, Iko is almost a direct contrast to the shy and introverted Teru. She is described as bright, cheerful, friendly, and very outgoing, possessing a natural charisma that allows her to make friends easily and draw attention to herself. She is proactive and often the one to initiate outings, pushing Teru out of her comfort zone. Her caring nature extends to a strong sense of responsibility, particularly towards children; having raised her younger brother largely on her own, she is exceptionally good at taking care of them and can calmly handle situations involving lost or distressed children. However, beneath this sunny exterior lies a deep-seated emotional turmoil. Iko carries immense guilt over the deaths of her parents, feeling that she failed to save them. This self-hatred, combined with the public criticism directed at Shy for the roller coaster accident, festers inside her.

This internal darkness makes Iko a prime target for the antagonist Stigma, who uses rings to corrupt people by amplifying the negativity in their hearts. When Stigma forces a ring onto her, Iko becomes the first person to fully unleash his power, transforming into a monstrous, corrupted form. In this state, she gains immense strength, speed, and durability. Her appearance changes dramatically, with her hair turning dark, her sclera becoming black, her right arm and leg taking on a sharp, crystalline quality, and a horn protruding from her forehead. She can manifest her heart's power as crystal-like weapons to attack. It is during this confrontation that Teru, as Shy, realizes Iko's hatred is not directed outward at her, but inward at herself. By reaching out with empathy and taking the brunt of Iko's attacks, Shy is able to free her from Stigma's control. This event becomes a turning point for both of them, and Iko emerges with a much healthier psychological state.

Following her rescue, Iko becomes Teru's closest friend and the only person who knows her secret identity as the hero Shy. In this role, she is an unwavering pillar of emotional support. She constantly encourages Teru, cheers her on during difficult battles, and acts as a confidante. Her motivation shifts from a painful sense of helplessness to a strong conviction to help Shy in any way she can, even without any superpowers of her own. She feels useless when she cannot assist, but her presence and words often provide the crucial emotional trigger that Teru needs. For instance, during a grueling test fight against the hero Stardust, Iko's desperate encouragement from the sidelines helps Shy awaken a new level of her flame abilities. Her unwavering faith in her friend makes her an invaluable, non-powered ally to the hero.