Isabel O'Sullivan is one of the twin protagonists of the anime. She and her identical twin sister, Patricia, are students who come to St. Clare's, a girls' boarding school, after their parents decide that a change of environment is needed for them.

Physically, Isabel has dark brown hair which she typically wears down, kept in place with a pink hairband. This style contrasts with her sister Patricia's ponytail and serves as one of the few visual differences between the two. She has deep blue eyes.

In terms of personality, Isabel is the quieter, more reserved, and more feminine of the two sisters. While initially resistant to the idea of attending St. Clare's, she adapts to her new environment faster than her more hot-headed sibling. She is a studious and diligent student who values organization and generally follows the school's rules. Isabel has a reputation for being a nice, down-to-earth girl who possesses a great deal of empathy for others, and she is often described as well-liked by her peers. She frequently smiles and has an angelic, gentle demeanor. As the more responsible twin, she sometimes disagrees with Patricia's rebellious and aggressive behavior, though she remains deeply loyal to her.

Isabel's primary motivation throughout the story is to navigate life at St. Clare's successfully. After her initial unhappiness, she becomes motivated to build friendships, succeed in her studies, and help her sister adjust as well. She is driven by a sense of fairness and compassion, often acting as a mediator and standing up for what is right. Her role in the story is to serve as a grounding presence for her sister Patricia and as a caring and reliable friend to the other girls at the school, including classmates like Alison and Janet. She helps to resolve conflicts and contributes to the overall harmony of the student body.

Key relationships are central to Isabel's character. Her most important bond is with her twin sister, Patricia. The dynamic between them is classic, with Isabel as the calm and sensible one to Patricia's impulsive and wild nature. Despite their differences and occasional clashes over Patricia's rule-breaking, their loyalty is unwavering, and they often support each other through various challenges. She also forms strong friendships with other students. She shows notable compassion for classmates who are struggling or being mistreated, such as Doris, and her relationships with authority figures, like the French teacher Mam'zelle, evolve from initial tension to mutual respect.

Isabel experiences significant character development. At the beginning of the series, she shares her sister's snobbish and petulant attitude, declaring that she hates St. Clare's. Over time, she mellows considerably, embracing the school's spirit and becoming a responsible, empathetic, and well-integrated student. This growth includes moving away from the mischievous schemes she initially participated in, such as the twin switch to help Patricia avoid punishment, and becoming a more conscientious individual.

Regarding notable abilities, Isabel is academically inclined and is considered a smart student. She excels in subjects like geography, putting a strong focus on her grades. While she participates in school sports like lacrosse, she is competent but not as naturally skilled or passionate about it as her sister Patricia. Her true strengths lie in her social and emotional intelligence, including her ability to empathize with others, resolve conflicts, and remain a stabilizing influence within her group of friends.