Haruka Takayama is one of the two main protagonists of her story and the best friend of Yuu Sonoda. The two girls have been inseparable since they met in junior high school, but their close bond is tested when they enter high school and are seated on opposite sides of the classroom. Haruka is deeply affected by this physical separation, far more than Yuu is, as it sparks an intense fear that her most important relationship might change. This anxiety is the catalyst for the central dynamic of their relationship, as Haruka proposes they do something special together to ensure their connection remains unique and irreplaceable.

In terms of personality, Haruka is best described as the extremely jealous and clingy type, a characterization that even Yuu uses to describe her. She is sweet-natured and generally demure, but she possesses a more aggressive and bold side when it comes to her feelings for Yuu. Haruka is the more emotionally sensitive and romantic of the pair, frequently losing herself in fantasies about Yuu in various romantic or lewd situations. Her jealousy often surfaces when Yuu pays attention to other friends, leading Haruka to feel depressed or anxious about their standing. Despite being academically gifted and book smart, Haruka is notably clueless when it comes to understanding the nature of romantic love. She often struggles to differentiate her romantic feelings for Yuu from other kinds of affection, which becomes a source of internal conflict and a recurring theme in her development.

Haruka's primary motivation throughout the series is to maintain and deepen her special bond with Yuu. She is the one who often initiates their secret kissing, using it as a way to reaffirm their relationship and assuage her own insecurities. Her role in the story is to drive much of the romantic progression, as she is constantly seeking ways to feel closer to Yuu, from sharing phone calls to finding excuses to be alone with her. While she is often the initiator, her fantasies are typically more elaborate than her actions, and it is frequently Yuu who surprises her with a more bold or unexpected response.

Haruka's key relationship is, without question, with Yuu Sonoda. This relationship evolves from an intense, possessive friendship into a romantic partnership. The two girls share a unique physical intimacy, often kissing in secret to solve their interpersonal issues or simply to express their affection. Haruka's love is genuine, and she is often the one pushing for a deeper commitment, although she is simultaneously confused by the nature of her own feelings. Another significant relationship, though more one-sided, involves Yuu's older sister, Mitsuki Sonoda. Mitsuki, the student council president, becomes suspicious of Haruka and Yuu's relationship and eventually develops a crush on Haruka herself, leading to frequent comedic delusions about being intimate with her.

Throughout the narrative, Haruka shows clear development from a girl plagued by insecurity and confusion over her feelings to someone who gains a better, though not complete, understanding of love. A major point of her growth is her determination to comprehend how romantic love differs from friendship, even after another character confesses feelings for her. By the end of the manga, her relationship with Yuu solidifies as they become an official couple. Furthermore, during her third year of high school, she matures into a leadership role, becoming the student council vice president.

Regarding her abilities, Haruka is portrayed as a good student who excels academically, to the point where others rely on her to copy her homework. Her most notable personal trait is her incredibly vivid imagination, as she constantly fantasizes about romantic and intimate scenarios with Yuu in almost any situation. Physically, she is self-conscious about her large chest size, which is noted to be a D cup, though she would prefer it to be smaller, often claiming it is a C cup.