Karen Fujisaki serves as the treasurer of the student council at Seifu High School. Her physical appearance is notable for being petite, with blonde or yellow hair styled into long twintails that reach past her hips, paired with brown eyes. She is almost always seen wearing her school uniform with stockings.

The core of Karen Fujisaki's personality is defined by an extreme and unwavering devotion to the student council president, Ui Wakana. This interest is frequently described as perverted or intense, manifesting in ways that drive much of her behavior throughout the story. Her primary motivation stems from this obsession, which places her in direct opposition to Hayato Izumi, the sole male member of the student council and Ui's romantic partner. Karen's feelings toward Hayato are characterized by murderous jealousy, and she regularly finds herself in conflict with him as a result.

Despite this intense animosity, her character is noted to show signs of development as the narrative progresses, with her hostile jealousy reportedly mellowing in later chapters. Her role in the story is largely defined by these relationships, acting as a source of both antagonism and comedic tension within the student council's dynamics. In her capacity as treasurer, she is a formal member of the council, but her personal agenda consistently revolves around Ui. One of her most frequently cited notable abilities or traits is her propensity for extreme physical reactions to her obsessions, including a severe nosebleed triggered simply by seeing Ui's mother dressed in a school uniform. Additional background details include the meaning of her name; Karen combines characters that can signify "possible" and "sympathy," while Fujisaki translates to "wisteria blossom".