Hotaru Imai is a central figure in her story, known for being the exceptionally intelligent and rational best friend of the main protagonist. She is a young girl, first introduced at the age of ten, with a short, mature black hairstyle and distinctive purple eyes. Her physical appearance is described as quite pretty, though her reserved and serious demeanor is what most people notice first. She is a student in the elementary division of a special academy, where she holds the high rank of a three-star elite and belongs to the Technical Class, a placement that suits her unique abilities.

Her personality is marked by a calm, logical, and often emotionally detached exterior. Hotaru rarely shows her feelings, preferring to analyze situations with a cool head and speak her mind bluntly, which can come across as cold or even arrogant. She has a notorious greedy streak and a love for large amounts of food, often making comical demands for payment in snacks. Despite her sour expressions and apparent laziness when it comes to ordinary tasks, she possesses a sharp, foul mouth and is not above using underhanded tactics like blackmail to get what she wants. Her favorite target for this is a shy classmate named Ruka Nogi, whose embarrassing photos she sells to his fan club.

Beneath this tough and mercenary exterior, however, lies a deeply caring and fiercely protective individual. This is most clearly seen in her relationship with Mikan Sakura, her best friend from her hometown. Unlike others who are put off by Mikan’s childish energy, Hotaru sees past it, just as Mikan sees past Hotaru’s coldness. She has a peculiar way of showing affection that involves constant teasing and physical correction, famously stating that she finds a crying Mikan cuter. Her hobby is essentially bullying the people she likes, which includes Mikan and the boys who have crushes on her. Yet, she is the first to defend Mikan from any genuine threat, making it clear that she is the only one allowed to make her friend cry. This devotion runs so deep that she admits Mikan’s smile is the only thing that truly brightens her mood.

Hotaru’s background explains her guarded nature. She has an older brother, Subaru Imai, who was taken to the academy years before her. Witnessing this, her parents spent years moving the family to keep the academy from claiming her as well. Eventually, to save her financially struggling elementary school from closing, Hotaru made a deal to enroll in the academy, using the conditional money to pay her old school’s debts. Upon arriving, she initially isolated herself, using her inventions as a shield to avoid forming attachments until she could earn the Best Student Award and return home. Her brother is also at the academy, and their relationship is a complex mix of insults, sibling rivalry, and unspoken care for one another. They avoid direct contact, yet she craves his attention and he is willing to sacrifice everything for her safety.

Her role in the story is triggered by her departure from their village, which prompts Mikan to follow her to the academy. After Mikan’s arrival, Hotaru begins to soften, allowing her natural dedication to her friends to emerge. She consistently volunteers for dangerous missions to help her classmates, offers strategic advice during conflicts, and uses her inventions to protect those she cares about. Her development is most profound in the later arcs, where her loyalty overrides her rational mind. When faced with the potential death of a friend, she adamantly refuses to accept it and plans a radical solution that involves traveling back in time to alter fate, fully aware that the punishment for such an act is severe and could result in her being erased from existence or trapped in time. Her sacrifice is a testament to her belief that her friends deserve happiness.

Her most notable ability is her Invention Alice. This power allows her to create an endless variety of functional devices, often designed to look like animals. Her inventions are highly sought after by major corporations, which contributes to her financial resources and high rank at school. Her creations range from whimsical modes of transport, like duck flying scooters, to practical tools such as infrared goggles, communication devices, and defensive items like horse hoof gloves. Her genius also allows her to temporarily replicate the alices of her friends through her inventions, making her an invaluable asset in any crisis. She is frequently seen breaking the fourth wall to explain her inventions directly to the audience, while her classmates simply view this as her talking to herself. By the end of her known story, Hotaru’s fate is to be lost in time with her brother as a consequence of changing the past, her existence fading from the memories of everyone she left behind, save for a lingering promise to her best friend that she intends to keep.