Satsuki Kirishima, Takumi’s childhood friend and neighbor, has shared a bond with him since their early years, growing up alongside her older sister Mizuki. Their dynamic thrives on casual, unguarded camaraderie rather than familial ties, marked by her habit of teasing him during daily routines—whether walking to school or slipping into his room through the window, her preferred entryway. This playful banter mirrors her lively, forthright personality, free from hesitation or shame in voicing her thoughts.

Despite their friendship, Satsuki and Takumi share a physical intimacy, including their first sexual experience—a secret she guards to spare Mizuki’s feelings. She harbors no regret, deeming it inconsequential, and continues interacting with him as before. Her choices stem from a mix of sisterly loyalty and personal candor, prioritizing genuine emotional connections over societal expectations.

Raised alongside Mizuki and Takumi, her interactions with them are steeped in years of shared history. New family members in Takumi’s household leave her behavior unaltered, though they frame the evolving dynamics around her. Satsuki’s steadfast outlook on relationships endures, emphasizing her pragmatic acceptance of secrecy when necessary to preserve the bonds she values.