Chiwa Harusaki is a central character known for being the childhood friend of the protagonist, Eita Kidou. She is a fifteen-year-old girl with a fair-skinned complexion, bright orange hair usually worn down except for two small sections tied with light-green ribbons, and violet-colored eyes. Her petite stature and these small ponytails, which resemble dog ears, have earned her the nickname chihuahua from her friends.

Chiwa has a friendly, boisterous, and playful personality, often acting with high energy and a straightforward openness about her feelings. This energetic nature is paired with a possessive streak regarding Eita, leading to intense jealousy whenever other girls, particularly Masuzu Natsukawa, get close to him. Initially, at the start of high school, she wanted to become popular with other male students, but she eventually realized that her true feelings were only for Eita, which led to an emotional breakdown and her subsequent decision to stop trying to impress others. She is extremely fond of meat in all its varieties, which is her favorite food.

Chiwa's personal history is marked by a significant tragedy that shapes her present circumstances. She practiced kendo from the first grade and was a promising athlete with dreams of competing at a national level. However, during a summer in her third year of junior high school, she was involved in a traffic accident that severely injured her lower back. Despite undergoing a major operation and intensive rehabilitation that allowed her to recover for daily life, her injuries were too critical for her to continue with kendo, forcing her to give up the sport she loved. This event is the very reason Eita decided to pursue a medical career, as he vowed to study hard to become a doctor to help heal her injury.

Within the story, Chiwa's role is defined by her long-standing relationship with Eita. As his neighbor since before elementary school, she has always been deeply attached to him, frequently declaring E-kun, I love you!. Eita, however, initially dismisses these declarations as merely friendly expressions of gratitude and tells her that a childhood friend could never become a girlfriend. Her motivation is thrown into sharp relief when the school beauty, Masuzu Natsukawa, returns from overseas and claims to be Eita's girlfriend, a fake relationship that Chiwa is unaware of. This provokes her jealousy and competitive spirit, leading her to join a club to compete with Masuzu and try to win Eita's affections for herself. Despite their quarrels, she ultimately comes to befriend Masuzu. She also maintains good terms with the other heroines, Himeka Akishino and Ai Fuyuumi, despite their shared rivalry.

Chiwa undergoes notable development, particularly concerning her competence and her relationship with the protagonist. A running gag is that she is a terrible cook, but inspired by Eita's cooking, she works hard to improve her skills over time. Her emotional journey comes to a head in a significant moment where she confesses her love to Eita and kisses him. This finally forces him to reconsider his view of her, moving from seeing her only as a sister to seeing her as a potential love interest. Her most notable abilities were in kendo, but due to her accident, she can no longer participate in the sport, though she has been known to instinctively put her former skills to use in rare situations. Her daily abilities remain limited to her ordinary life as a student, with her primary strengths being her determination and her unwavering, if initially unrequited, love for her childhood friend.