Itsuki Midoriba is a student at Verbena Academy and a classmate of the main protagonist, Rin Tsuchimi. Possessing a sharp intellect and consistently high exam scores, he is considered one of the most intelligent students at the school. Physically, he is tall and handsome, with brown hair and eyes, and is known for his charismatic and attractive appearance.

Despite his academic prowess, Itsuki's personality is largely defined by his frivolous and arrogant demeanor. He refers to himself using the grandiose and self-aggrandizing term oresama, a highly confident way of saying "I" in Japanese. His primary interest is pursuing and flirting with women, and he spends much of his free time checking them out. This behavior is a constant source of comedy in the series, often leading to confrontations or punishments, particularly from his homeroom teacher, Nadeshiko Benibara, who frequently gives him detention for his advances.

Beneath his playboy exterior, Itsuki possesses a shrewd and insightful nature. He is often the one to explain complex situations to his friends and, in critical moments, provides the brilliant solutions that drive the plot forward. For instance, during Primula's story arc, his strategic thinking is instrumental in preventing a major crisis and helping Rin restore his memories. When speaking seriously, his first-person pronoun shifts from the arrogant oresama to the more standard ore, signaling a moment of genuine sincerity and intelligence.

Itsuki's relationships are central to his character. He shares a deep, albeit complicated, friendship with Rin Tsuchimi. The two met in middle school when Rin bravely helped Itsuki and his childhood friend fight off a group of bullies. Since then, Itsuki has acted as a reliable, if troublesome, confidant. He openly envies Rin for being surrounded by beautiful girls and frequently jokes that he will take care of any Rin does not choose, yet in times of genuine need, he is an unwavering source of support and wisdom. He also holds a notable admiration for Kaede, constantly praising her beauty and personality, though his attempts to hug her are regularly thwarted by Rin.

One of his most significant and enduring relationships is with Mayumi Thyme, his childhood friend. Their interactions are characterized by constant bickering, often revolving around comments about Mayumi's chest size. However, it is revealed that Itsuki actually holds romantic feelings for Mayumi, and his outward insults are merely a way to get a reaction from her. He is also a frequent subject of his teacher, Nadeshiko Benibara, whose conversations with him are a recurring comedic highlight, usually ending with him being assigned an absurd punishment or a passionate lecture about his studies.

Throughout the series, Itsuki serves as the comedic foil and a powerful, unexpected ally. While he appears to be nothing more than a womanizing slacker, his high intelligence and moments of level-headedness make him an indispensable part of the group. He represents the archetypal "bad friend" who is, in reality, the most reliable person in a crisis. His development is subtle, as his core personality remains constant, but his role shifts from pure comic relief to a serious problem-solver when the narrative demands it, proving that his sharp mind is a genuine asset beneath the layers of arrogance and lechery.