Hikaru Hiyama is one of the central characters in the love triangle that drives the story. She is a middle school student who is two years younger than the protagonist, Kyosuke Kasuga, and his primary love interest, Madoka Ayukawa, with whom she shares her birthday of November 15th. Physically, she is recognizable by her bobbed chestnut hair and a distinctive tear-shaped mole near her right eye.

Upon her first appearance, Hikaru is presented as an unpleasant brawler and a tougher talker than her friend Madoka, often seen engaging in delinquent behavior. However, this initial roughness is quickly revealed to be a superficial front. Once she becomes friends with Kyosuke, her true personality emerges: she is fundamentally kind, sweet, energetic, and fiercely loyal to those she cares about. This core persona is marked by a childlike innocence, partly attributed to her status as an only child who has been somewhat sheltered. When extremely excited, she has a tendency to revert to a form of baby talk, a trait presented as endearing. Her enthusiasm and extroverted nature make her a natural atmosphere maker who brightens the mood of any group she is in. Despite her cheerful disposition, she can be naive and is not particularly skilled academically, sometimes requiring remedial lessons.

Hikaru's primary motivation and the driving force behind her actions in the story is her deep affection for Kyosuke Kasuga. Her feelings are kindled after she accidentally witnesses Kyosuke make an extraordinary, seemingly impossible, long-distance basketball shot, an act he accomplished using his family's secret psychic powers. Impressed by what she believes to be pure athletic prowess, she falls for him immediately and decisively. She demonstrates her forward and uninhibited personality by aggressively pursuing him, famously calling him Darling from the moment she becomes interested, even before learning his name, and frequently greeting him with enthusiastic embraces. While her love for Kyosuke is intense and sincere, it is also portrayed as somewhat naive and idolizing, as she is initially drawn to a single impressive act.

Her role in the story is as the third point of the primary love triangle. She is the best friend and near-constant companion of Madoka Ayukawa, a relationship that predates the events of the series. Their friendship was sealed when a pre-teen Madoka helped a young Hikaru retrieve a stolen medallion from a bully, making Hikaru one of the few people who was never afraid of Madoka's tough reputation. Hikaru deeply admires Madoka, viewing her almost as an older sister. This dynamic creates the central dramatic irony: Hikaru is in love with Kyosuke, who is secretly in love with Madoka, who in turn has developed feelings for Kyosuke but is torn by her loyalty to Hikaru. Hikaru remains oblivious to the true romantic tension between her two closest people for most of the story, acting as a cheerful and innocent obstacle to the mutual attraction between Kyosuke and Madoka, who often finds herself encouraging the relationship between Kyosuke and Hikaru out of a sense of duty and friendship.

Kyosuke Kasuga cares for Hikaru deeply, but his feelings for her are predominantly platonic and protective, more akin to the affection one would have for a younger sister rather than a romantic partner. This mismatch in feelings is the source of much of the story's conflict and Kyosuke's indecisiveness, as he does not want to hurt Hikaru by rejecting her. Another key relationship is with Yusaku Hino, a childhood friend of both Hikaru and Madoka. Yusaku is hopelessly in love with Hikaru, but she generally treats him with a lack of interest that can border on cruelty, seeing him more as a nuisance or a servant than a potential romantic interest.

Over the course of the narrative, Hikaru undergoes significant emotional development, primarily moving from cheerful obliviousness to painful awareness. She remains ignorant of Kyosuke and Madoka's true feelings for each other until late in the story, largely because those around her keep the secret to protect her from being hurt. When she finally learns the truth, triggered by a seemingly offhand comment, the revelation is shattering. She realizes that Kyosuke has always loved Madoka, the girl who gave him a red straw hat that he still keeps. Her development culminates in a moment of mature heartbreak and acceptance. While she experiences intense emotional pain and anger, ultimately slapping Kyosuke when he tries to apologize, she demonstrates remarkable grace by not lashing out to destroy their happiness. She ultimately expresses that despite her own feelings, she wishes for the happiness of both Madoka and Kyosuke, whom she deeply cares for. In the concluding parts of the story, her character arc focuses on the painful process of letting go of her first love and accepting the changed reality of her relationships with the two most important people in her life.

One of Hikaru's most notable abilities is not supernatural but social. She is an incredibly loyal friend, having stood by Madoka for years when few others would. Her energetic and affectionate nature allows her to form strong bonds quickly and bring light to serious situations. She also possesses artistic talent, having shown a particular skill for drawing, which contrasts with her lack of aptitude in academics and singing. However, her most defining characteristic is her single-minded and vocal devotion to Kyosuke, expressed through her unwavering use of her pet name for him and her physically affectionate antics.