Nagisa Minase is a high school student with short blue hair tied with a blue ribbon, blue eyes, and a smaller stature. She wears her school uniform with a blue bowtie and features a noticeably larger bust. Initially plain, her personality combines sweetness, persistence, selflessness, consideration, and pacifism. She works tirelessly to overcome perceived shortcomings in areas like singing or sports. Despite her considerate nature, she employs underhanded tactics or cheating when necessary to protect her romantic relationship, including blackmail as a last resort without sinister intent. She reliably maintains secrets about her relationship and demonstrates honesty under pressure.

Inspired by witnessing Mukai Naoya's dedication to his girlfriend Saki Saki, Nagisa underwent a significant transformation. She dedicated extensive effort to practicing sports and housework, changed her hairstyle, and switched from glasses to contact lenses upon entering high school. This focus caused her academic performance to decline. She later confessed to Naoya. As he was already dating Saki, he proposed dating both simultaneously. Nagisa accepted and helped convince Saki to agree, subsequently moving in with them. Her dedication to winning Naoya's heart led to skipping many classes, making her a stranger in her own class.

Nagisa is deeply moved by Naoya's dedication and works hard to win his affection. She accepts the non-monogamous relationship structure to remain close to him, sharing her first kiss with him later as an expression of gratitude and encouragement. With Saki Saki, Nagisa transitions from rival to friend, objectively admiring Saki's role as Naoya's childhood friend. Their dynamic evolves into mutual romantic affection, which surprises Nagisa. Regarding Hoshizaki Rika, Nagisa initially perceives her as a threat and works to keep Naoya away, later adopting a more passive stance. Interactions with Kiryuu Shino begin with Shino's disapproval due to Nagisa's impact on Saki's relationship, but their rapport gradually improves.

Her name "Minase" combines "water" (水) and "swift current, rapids" (瀬), while "Nagisa" translates to "beach, shore, water's edge" (渚). She becomes the caring caretaker within the relationship and demonstrates a willingness to prioritize Saki's connection with Naoya over her own. Her first kiss with Naoya occurs significantly later than with other heroines.