Chika Igarashi, the younger stepbrother of Iroha Igarashi, shares a familial bond forged since childhood, having been raised alongside her from age five. Clinging to an unrequited passion, he fixates on Iroha despite her consistent rejection, driven by a consuming need to monopolize her affections. His possessiveness manifests in confrontations with her romantic partner, Hikari Tsutsui, whom he deems unworthy—escalating to physical altercations and demands to sever their relationship.

When Iroha undergoes brain surgery and develops retrograde amnesia, erasing memories of Tsutsui and their history, Chika initially exploits her fractured memory, omitting details of her past to isolate her from former connections. Yet her instinctual pull toward familiar places compels him to reluctantly disclose the truth, conceding his inability to fully dictate her life.

Years later, after separation during Iroha’s recovery abroad, Chika arranges her return to Japan, inadvertently paving the way for her reunion with Tsutsui. This gesture hints at a conflicted shift from manipulation to muted recognition of her independence, though his earlier actions remain rooted in self-interest.

His background includes unspecified studies overseas, while his name carries layered symbolism: "Igarashi" blends "fifty" and "storm," and "Chika" derives from "thousand" and "summer." The character exists exclusively within the anime and manga narratives, absent from the live-action adaptation.