Mao Amatsuka is the president of the GJ Club and the eldest of the three Amatsuka sisters. She is a high school student with a notably small and petite build, so much so that she is frequently mistaken for an elementary school student. She has wavy ginger hair that can shift slightly depending on her mood and orange or brown eyes. Despite her youthful appearance, she is a year older than the protagonist Kyoya Shinomiya and holds the position of club leader.

Her personality is forceful, short-tempered, and dominant. She has a habit of biting Kyoya when she is bored, angry, or embarrassed, and she frequently picks on him as a way to pass the time. She is impulsive and often acts as a troublemaker within the club, with many situations arising from her complaints about being bored. She is openly selfish and has little patience for things she dislikes. One of her most notable dislikes is romantic or kiss scenes in any media, and she cannot stand the idea of kissing even on the forehead or cheek to the point of carefully choosing which books or shows to consume. She prefers shonen manga over romance, which contrasts with her sister Megumi's more feminine tastes. Her personality carries a tsundere quality, as she masks any affection or embarrassment with physical aggression.

Her motivations center on maintaining the club's status quo and her position as its leader. She values the daily routine and comfortable chaos of the GJ Club and is protective of how things work within that space. She is not particularly driven by academics or external achievements but by the enjoyment she derives from the club's free time and interactions with the other members.

In the story, Mao is the commanding president of the GJ Club and one of the central characters. She sets the tone for the club's activities, whether involving pranks, competitions, or simply lounging around. She is the one who gives Kyoya the nickname Kyoro, which sticks throughout the series. Her role involves driving much of the comic conflict due to her impulsive and demanding nature.

Her key relationships include her two sisters. She is the older sister of Megumi and Seira Amatsuka, though she is shorter than both, a recurring point of humor. Megumi, the middle sister, acts as a caretaker and calming influence, often stepping in to mediate Mao's outbursts. Seira, the youngest, is more observant and tends to tease or observe Mao from a distance. Mao shares a long-standing friendship with Shion Sumeragi, the genius chess player in the club. She also has a close bond with Kirara, the athletic club member. Her most dynamic relationship is with Kyoya Shinomiya, the only male club member. She frequently bites him, uses him as a footrest, and bosses him around, but she also relies on his presence and patience. Their dynamic is tsundere in nature, as she hides any deeper feelings behind aggression. She cares for him in her own way, though she would not admit it openly.

Her development is tied to the passage of time and the reality of graduation. As a third-year student, she must eventually face leaving the club and the end of its daily routine. Her arc involves coming to terms with growing up and moving on while still holding onto her spirited and dominant nature. She struggles with the bittersweet feeling of her time in the clubroom being limited, and she learns to pass on the club's legacy while ensuring its atmosphere remains unchanged.

Her notable abilities are less about traditional skills and more about her personality traits. She possesses a strong leadership presence despite her small stature, and she can command the attention of the other club members. She has a talent for creating trouble or entertainment out of boredom, often dragging others into her schemes. She also has a habit of talking in her sleep when she has bad dreams. Her physical strength is not particularly notable, but she is quick to use her teeth as a form of discipline or expression.