Imadori Kyôsuke is a student in class 2-C at Yagami High School, recognizable by his spiky blonde hair and droopy eyes that often give him a bored or disinterested expression. He is known by the nickname "the Hummingbird" and is a member of a group called the Five Elite Tigers, having pledged to be sworn brothers with fellow student Ganji Nishimoto in what they call "the Oath of the Video store". His family owns a beauty salon, and he has a pet dog.

Personality-wise, Imadori is defined by his laid-back, carefree, and self-centered attitude. He is notoriously perverted, with his primary interest in women being focused almost exclusively on those with larger breasts, specifically a D cup or larger. This obsession drives much of his behavior, leading him to constantly flirt with female classmates and spend his free time calling girls on his cell phone. Despite this shallow fixation, he possesses a surprising amount of charm and can act cool and suave when necessary, which allows him to secure numerous dates. He is also a devoted fan of a children's television show called Dojibiron, a trait that highlights his immature and playful nature. His antics are fueled by a lack of common sense and a seeming fearlessness when it comes to danger, whether he is provoking a school delinquent or irritating a physically powerful classmate, often with painful results.

Imadori's most notable ability is his seemingly supernatural talent for accurately guessing a woman's bra cup size with just a glance, a skill occasionally referred to as the "Imadori Eye". While not a primary focus of the series, he has shown some athletic ability, particularly in basketball where he played as a forward for a team called "The Outlaws" and demonstrated a surprisingly high jump that allowed him to perform slam dunks. He also possesses decent physical strength, enough to grapple with the skilled martial artist Haruki Hanai when motivated by his perverted goals.

In the story, Imadori begins as a minor supporting character but becomes a major source of comedic relief. He interacts with a wide range of characters, creating humorous dynamics due to his single-minded obsession. His key relationships are primarily with several female characters. He is constantly pursuing Mikoto Suo, drawn to her athletic build and large chest, though his advances are invariably met with physical violence. He similarly enjoys provoking the short-tempered Lala Gonzalez at her workplace, WAS Burger, which also usually earns him a beating. His most significant relationship, however, develops with Karen Ichijo. Initially, he is afraid of her due to her monstrous strength and her smaller chest size, which falls outside his usual interests. Over time, they begin working together and go on dates, and despite his oblivious nature and continued flirtations with other girls, he shows moments of genuine care for her, such as when he rescues her from a club. This relationship evolves in the manga's sequel, School Rumble Z, where, after a series of misunderstandings, he gives Karen a promise ring, signifying a deeper, albeit still clumsy, commitment. He also shares a friendship with Karen's younger brother, Kosuke, based on their mutual love for Dojibiron.

Imadori shows some development throughout the series. While he remains a pervert at heart, his relationship with Karen forces him to occasionally act beyond his superficial desires. He demonstrates a more sensitive side when he tries to make Lala smile at her job or when his concern for Karen overrides his typical self-centeredness. His arc culminates not in a complete personality change, but in a growing, if still unspoken and imperfect, affection for Karen, showing that his character is capable of more than just his one-track obsession.