Yūya Kanzaki becomes Mitsuki Kanzaki's stepbrother when their parents remarry and relocate overseas, leaving the two alone to cohabitate. He is a well-intentioned high school student determined to forge a positive sibling bond with Mitsuki. However, his upbringing devoid of maternal guidance and minimal exposure to female relationships severely hinders his capacity to comprehend or empathize with Mitsuki's emotions and viewpoints.

Yūya routinely misreads Mitsuki’s behavioral signals and physical cues due to his social inexperience. This leads to incidents like forcibly escorting her to the school clinic when she urgently needs the restroom, misinterpreting her distress as illness despite her protests. His pattern of flawed assumptions extends to other contexts, such as mistaking the chastity belt imposed on Mitsuki by the ghost Hiyori Kotobuki for evidence of a "lesbian cosplay couple" dynamic, further straining their interactions.

Throughout their shared living arrangement, Yūya’s attempts to connect with Mitsuki frequently misfire, though incremental progress unfolds. Mitsuki maintains initial coldness and distance, partly rooted in her fraught history with her biological father. Yet subtle shifts emerge, including moments where Mitsuki exhibits jealousy over Yūya’s exchanges with his childhood friend Yukina Kiritani. By the series' conclusion, Mitsuki begins addressing him as "onii-chan," signaling a softening of her resistance and fragile acceptance of their fraternal tie.

In ancillary OVA content, Yūya overlooks Mitsuki’s birthday amid Christmas festivities. After realizing this error through a discussion with their mother, he strives to compensate, revealing heightened sensitivity to her feelings.

Yūya’s narrative arc revolves around his persistent struggle to fulfill his role as an older brother despite enduring gaps in emotional perception. Though his actions consistently originate from a supportive intent toward Mitsuki, their unintended repercussions propel much of their evolving dynamic.