Yū Mitarai is a first-year Mechanical Engineering student at Izu University, where he is a classmate of the protagonist, Iori Kitahara. He is a young man of about average height, standing at 165 centimeters, with a slender build and a generally handsome appearance. His most distinctive physical features are his sharp, narrow eyes that often appear to be closed, and his black hair, which is accented with partial blond highlights, suggesting it has been dyed.

Initially, Mitarai comes across as a fairly typical college student, but he soon reveals a personality that is far more frivolous and impulsive, particularly in his attitude toward women. He frequently acts as a playboy, trying to flirt without much consideration for the consequences, and shows little tendency to genuinely reflect on his mistakes. When faced with trouble, he is known for resorting to quick, often absurd solutions to extricate himself from difficult situations, such as making an impromptu marriage proposal to defuse a volatile confrontation. Beyond his relationships with women, Mitarai is also highly susceptible to the influence of his peers, often getting swept up in their chaotic antics and schemes. His tendency to follow the crowd makes him a frequent participant in the wild escapades that define his university experience.

The central axis of Mitarai's personal life is his turbulent relationship with his childhood friend, Rie Oohashi. He was originally in a romantic relationship with her, but it became a recurring source of conflict. Much of the trouble stems from the machinations of his friends, particularly Iori Kitahara and Kōhei Imamura, who repeatedly sabotage the relationship for their own amusement. On one occasion, they fabricated a false rumor of infidelity, which caused a painful breakup. During the ensuing argument, Mitarai brazenly asked Rie to introduce him to her friends, an act of carelessness that provoked further anger and physical retribution. Later, after a misunderstanding where he was falsely accused of picking up women, he was subjected to a public humiliation that his peers called an "execution". In a desperate attempt to avoid punishment on another occasion when he was suspected of cheating, he proposed marriage on the spot, a move that ultimately led to him becoming completely controlled by Rie, a dynamic to which he appears to offer little objection. Despite the chaos, his relationship with Rie remains a constant, if unbalanced, presence in his life.

His role within his group of friends, which includes Iori, Kōhei, Shin'ichirō Yamamoto, and Hajime Nojima, is often that of a fellow instigator or a hapless victim of their collective mischief. He is easily drawn into their pranks and schemes, which frequently backfire and lead to his own suffering. In one notable anecdote that encapsulates his character, Mitarai attempted to cheat on a German language exam by hiding a cheat sheet in his pencil case. His plan failed spectacularly when the exam proctor required all pencil cases to be handed in before the test began. This incident, and others like it, became a legendary story among his peers, cementing his reputation for comic misadventure.

Despite his many blunders and his often irresponsible nature, Mitarai possesses a surprising and notable talent: he is highly skilled in the art of ikebana, or Japanese flower arranging. This ability stands in stark contrast to his otherwise chaotic and frivolous behavior, revealing an unexpected, refined side to his character. His development throughout the series is not so much a linear progression but a cyclical pattern of self-inflicted trouble, comedic suffering, and brief moments of using his wits to narrowly escape even greater disasters, all of which solidify his role as a source of entertainment and a memorable part of the university's colorful social landscape.