Kotaro Yuda shared a middle school history with Hana Arai and Alice Arisugawa, including a childhood friendship with Hana. As they grew older, he grew distant despite her persistent romantic feelings. On Valentine's Day, Hana presented him with a mock marriage license expressing her desire for permanence, which he rejected.

Following this, he distributed similar certificates to multiple other girls in their class, pursuing romantic engagements that caused emotional distress among classmates and shaped his school reputation.

An incident occurred when Hana, resentful over his actions, placed a bee inside his shirt collar. Unaware of his severe bee sting allergy, this act triggered a significant physical reaction, causing him to collapse in class. Student rumors swirled about potential fatal anaphylaxis, contributing to a school urban legend.

He transferred schools the same day as the bee sting. His abrupt departure fueled persistent rumors among students, including suggestions he had died and speculation about "four wives" being involved.

Later encounters with Hana and Alice confirmed his survival. During one brief meeting, he acknowledged knowing Hana placed the bee and explicitly stated he remembered the pain, cautioning her against repeating the act. His demeanor remained detached, showing minimal emotional engagement with Hana or her past feelings.

His personality is characterized as emotionally reserved and indifferent to the impact of his actions on others, evidenced by his rejection of Hana, his pattern of insincere romantic pursuits with classmates, and his dismissive attitude during the later encounter.