Aiki Hirose is a central character and the object of the protagonist’s affection. He is a fifteen-year-old high school student and classmate of Okuto Nakamura. Physically, Hirose is described as having wispy, chestnut-brown hair and big, round brown eyes with notably long eyelashes. He has light skin, despite Nakamura describing him as having a slightly tanned complexion, which is considered a design inconsistency. A notable physical trait is his short stature, a characteristic he is personally sensitive about.

Hirose’s personality stands in stark contrast to the shy and introverted Nakamura. He is an extrovert who has no trouble talking to anyone, making him a quintessential "insa" or popular type. His core nature is kind-hearted, friendly, and sweetly optimistic, with an honesty and cheerful demeanor that permeates his actions, sometimes to a childish extent. He is a member of the school’s health committee and is readily helpful to those in need. Despite his friendly exterior, his considerate nature can make it difficult for him to express his own less agreeable feelings. However, when provoked, he can become agitated, and his honesty has been known to lead him into trouble. He possesses an exceptional memory, a fact that often surprises those around him, particularly Nakamura. He has a habit of saying "Cool!" and "Crap...".

In the story, Hirose serves as the love interest and classmate of Nakamura, who is immediately smitten with him at their high school entrance ceremony. Initially, Hirose is unaware of Nakamura’s feelings and views him simply as a stranger or a distant classmate, though he acts friendly towards him as he does with everyone. As the narrative progresses, their relationship develops from mutual strangers to a reciprocal friendship. A key moment in their relationship occurs when Hirose confides in Nakamura, admitting that he is the first person who has ever listened to him vent, which signifies a growing trust.

Hirose’s key relationships extend beyond Nakamura. His closest friend is Oomori, with whom he has a long history as a childhood friend dating back to elementary school. He is also friends with Takeuchi, though Takeuchi’s personality often puts Hirose in uncomfortable situations. He greatly admires his teacher, Otogiri Sou, whom he considers "cool," especially after the teacher saved him from getting into trouble; this admiration is hinted at as possibly being a crush. Other friends include Mukai and a middle school friend named Matsumura Ko. Outside of school, he has a sister and lives with his parents.

Throughout the series, Hirose shows development in his ability to express his true feelings. Initially struggling to communicate his discomfort with friends who talk about girls or tell dirty jokes, he learns to be more open after his interaction with Nakamura, eventually helping Takeuchi confess to a girl named Yuuka. His development is also marked by his growing closeness with Nakamura, moving from not recognizing him to actively seeking his company and declaring that he thought they were already friends.

Notable abilities and personal details include his talent for imitating a machine gun and his exceptional memory. His favorite foods are meat and potato chips, while he dislikes cow and pig entrails. His hobbies include comedy in a vague sense, and he enjoys Western music and suspense movies. He loves dogs and, among sea life, crabs.