Kai Miura is a central figure in the story, known for his striking appearance and complex personality that often defies initial expectations. Physically, he is a tall teenage boy frequently described as beautiful, with sharp brown eyes and dyed blonde hair, which he lightened from its original black during middle school. His typical attire consists of the school uniform, often paired with a white zip-up hoodie and a baseball cap, and he is recognized for his two silver hoop earrings. His easily recognizable lemon-colored hair is a noted trait, and he carries himself with a distinct sense of style, having previously worked as a salon model which contributes to his fashionable appearance.

Kai’s personality is defined by a cool and aloof exterior. He is generally distant and does not engage with the many admirers he has at school, which contributes to his reputation for being brash or off-putting. Despite this cold demeanor, he possesses a strong sense of justice and is deeply observant of those around him. He is often seen holding a bottle of lemon soda, which he uses in a nonchalant manner to deter bullies. While he can seem sleepy or his moods can change, true warmth and sincerity are reserved for a small circle of close friends and, most significantly, Uka Ishimori.

The character's background is marked by family loss. His mother passed away when he was approximately three years old, and his father left home during the winter of his third year of middle school. Following this, Kai came to live in an apartment owned by his father, staying with his uncle. These experiences have shaped him into someone who values genuine connection and can be deeply affected by loneliness.

Kai’s primary motivation within the narrative is his desire to help Uka Ishimori break free from her crippling social anxiety and self-doubt. Unlike others who ignored or bullied her, Kai actively encourages her to stand up for herself, pushing her to change and become more confident. His personal aspirations extend beyond high school, as he aims to work in international business and marketing, a goal fueled by a sentimental wish to one day give Uka's father a nice suit.

In the story, Kai serves as a catalyst for change, particularly for Uka. His role evolves from a protector who offers a helping hand when others avoid her, to a boyfriend and primary source of support. He has a notable ability to observe details and understand people, which he uses to guide those he cares about. His past includes a relationship with a girl named Serina Kanno during middle school, a former connection that later introduces tension and complexity into his current relationship. His closest relationship is with Tomoya Takamine, his childhood best friend to whom he is as close as a brother. Kai also takes on a mentorship role with a younger student named Sho Iteya, who initially admires Kai's rebellious side; Kai takes Sho into his home to help him straighten out his life, demonstrating his capacity for quiet, brotherly care.

Kai’s development is intertwined with his deepening relationship with Uka. Beginning as a guardian figure, he eventually falls in love with her, and they begin dating. Their relationship faces obstacles, including interference from others who try to come between them, but they consistently work through these challenges, growing stronger as a couple. Over time, his feelings solidify to the point where he and Uka make plans to move in together after graduating from high school. While he is fiercely loyal and supportive, his communication style can be stern and indirect, sometimes causing Uka to doubt his feelings, though his actions consistently demonstrate his commitment.