Minami Tenjin serves as the male deuteragonist in his series. A second-year student at Seishun Academy, he enjoys immense popularity, widely regarded as a school idol. His striking appearance features purple eyes and blond hair styled in a mullet, with bangs covering the right side of his face; this blond hair is later revealed to be dyed, his natural color being brown. He stands relatively tall, slightly shorter than Jirō. His typical school attire consists of an untucked blue button-up shirt worn open over a lavender undershirt, dark blue pants secured with a brown belt, and brown loafers.

Personality-wise, Minami exhibits a calm, cool demeanor and excels athletically in sports like basketball, volleyball, and track-and-field. He is also academically proficient, achieving high test scores. Friendly and respectful to everyone, including socially isolated students, he demonstrates humility by crediting others. His observant nature allows him to notice subtle emotional shifts, such as Akari Watanabe's initial crush on him and her later feelings for Jirō. However, he harbors significant personal struggles. Emotional trauma from a past romantic rejection has distorted his perspective on love, leading to a persistent, one-sided attachment that hinders new relationships. His strong desire to please others causes him to overextend, like volunteering for excessively demanding school tasks.

His family background involves a strained relationship with parents who impose high academic expectations. In rebellion, he dyed his hair blond and chose Seishun Academy against his father's wishes. A pivotal traumatic event occurred years earlier when he developed feelings for his tutor, Nozomi, who was also his older brother Minato's fiancée. After confessing and being rejected due to her engagement, he accidentally witnessed her and Minato in an intimate situation. This experience left him mentally scarred, cementing an unhealthy, unresolved attachment to Nozomi despite her marriage to his brother.

At Seishun Academy, Minami participates in the "Marriage Practical" program, partnered with Shiori Sakurazaka. They maintain a functional partnership, sharing household duties like cooking equally, though Shiori's initial anxiety around males impacted their early evaluation scores. Despite reaching the top "A-Rank" multiple times with Shiori, he chooses not to switch partners. His popularity made him the object of Akari Watanabe's affection during her first year; she developed a crush after he defended her against a false accusation. When Akari confesses during a school trip, he gently rejects her, revealing his past trauma with Nozomi. This conversation helps Akari realize her feelings have shifted toward Jirō. Minami states he views Akari like a younger sister and supports her pursuit of Jirō's affection.

His interactions with Jirō begin distantly, partly due to Jirō's jealousy and insecurity. Their relationship evolves during a sports festival when Minami, recognizing Jirō's observational skills during an injury, asks him to replace an injured student in a relay race. Initially reluctant, Jirō agrees, and their collaboration leads to victory, fostering mutual growth: Jirō learns to rely on others, while Minami accepts he doesn't need to shoulder burdens alone. Minami also works part-time at Starbox Coffee, where he has known coworker Shū Terafune for at least a year.

His emotional journey centers on the lingering effects of his unrequited love for Nozomi. He consciously distances himself from romantic possibilities, including Akari's advances and rumors about Shiori, due to this fixation. A moment where Shiori slaps his hand away triggers visible distress, highlighting his unresolved trauma. Despite this, he shows gradual progress by supporting Akari's emotional growth and forging a genuine friendship with Jirō. His decision to leave his family home during a tense Christmas dinner, defying his father's demand to cut ties with friends, further illustrates his struggle between familial expectations and personal connections.