Tsubasa Tanuma, a student progressing from Class 2-B to 3-A at Shuchiin Academy, first appears anonymously as "Kashiwagi’s Boyfriend" before his full identity emerges. His family ties link to grandfather Shouzou Tanuma, a hospital director, and a father continuing the medical legacy. Initially reserved in romance, he turns to Miyuki Shirogane for advice, mastering the "kabe-don" technique to confess to Nagisa Kashiwagi—a move that successfully wins her affection.

Embracing newfound confidence post-confession, Tsubasa revamps his image with dyed hair and piercings to match Nagisa’s tastes. His frequent student council visits, framed as seeking guidance, evolve into boastful anecdotes about their relationship, unsettling peers. Public displays of affection and candid discussions of intimacy mark their dynamic, yet his unwavering kindness draws Maki Shijo’s unnoticed admiration. Academically, he rises through study sessions with Nagisa, a top-ranked classmate.

Their relationship escalates when Nagisa’s high school pregnancy results in Tsubasa becoming a teenage father at seventeen, echoing his family’s trend of early parenthood. He later mentors Yu Ishigami, advocating the same "kabe-don" method Miyuki once taught him, perpetuating a cycle of romantic strategies.

His background includes a younger sister and a family steeped in medicine, though maternal details remain undisclosed. Narrative threads gradually unveil his full name and lineage across multiple chapters, resolving initial ambiguities around his identity through deliberate storytelling progression.