Hotaru Imai is a central figure in the narrative, primarily known as the best friend of the protagonist, Mikan Sakura. Physically, Hotaru is a young girl with short, straight black hair that gives her a mature appearance beyond her years, and distinctive purple eyes. At the start of the story, she is ten years old. Her demeanor is typically calm, logical, and reserved, often making her seem emotionally detached or serious compared to her more boisterous classmates. Despite this cool exterior, she possesses a sharp, sarcastic wit and is known for her foul mouth, though she rarely raises her voice.

Hotaru’s primary motivation is deeply rooted in her practical and protective nature. She transferred to the elite Alice Academy, a school for children with special abilities, not only because her family could no longer evade the government’s enrollment policies but also to secure a large sum of money to prevent her former elementary school from being demolished. Her goal to earn the "Best Student Award" is driven by the reward of temporary home visits, which would allow her to see her family and Mikan again. Within the story, she acts as the sharp, rational anchor to Mikan’s impulsive and emotional personality. Hotaru often uses her inventions to bail her friends out of dangerous situations or to discipline Mikan when she acts carelessly.

Her key relationships are central to her character development. The bond with Mikan Sakura is the most significant. While Hotaru frequently criticizes and teases Mikan, even going so far as to say she finds a crying Mikan cute, her actions consistently demonstrate profound care and loyalty. She famously declared that she is the only one allowed to make Mikan cry, and she has willingly put herself in harm's way to protect her, such as when she took a bullet meant for Mikan. Hotaru shares a complicated but caring relationship with her older brother, Subaru Imai. Both are reserved and often insult one another, yet they deeply care for each other’s well-being. She also enjoys subtly tormenting Ruka Nogi and Natsume Hyuga, both of whom have feelings for Mikan, often by taking embarrassing photos of Ruka to sell to his fan club.

Throughout the series, Hotaru undergoes significant development, shifting from a girl who intentionally isolated herself to focus on her goals to someone who openly values her friendships. Initially, she wore inventions to hide her face and distanced herself from peers, believing attachments would slow her down. However, her classmates noted that the first time she truly smiled was the day Mikan arrived at the academy. Her evolution culminates in a major act of selflessness: when Natsume dies, Hotaru defies the natural order and travels to the past to save him, fully aware that she would face severe "space-time retribution" for changing fate. This act causes her and her brother to be erased from everyone's memories and trapped in a perpetual loop through different time periods, demonstrating her ultimate willingness to sacrifice her own happiness for that of her friends.

Hotaru possesses the Invention Alice, a Technical-type ability that allows her to create unique, often animal-shaped, mechanical devices. Her inventions range from the practical, such as filming glasses and communication mufflers, to the absurdly combative, like the Baka Gun and Baka Cannon, which she frequently uses on Mikan. She is a genius inventor whose creations are sought after by foreign companies and classmates alike, earning her the elite Triple Star rank despite her young age. A notable and comedic aspect of her character is her habit of breaking the fourth wall to explain the mechanics of her inventions to the audience, a behavior that the other characters interpret as her talking to an empty space. Other notable traits include her voracious appetite, laziness, and a strong fixation on earning and saving money.