Identical twin Isabel O'Sullivan arrives at St. Clare's alongside her sister Patricia due to parental concerns about their behavior. She possesses dark brown hair and blue eyes. Isabel distinguishes herself from her sister by wearing her hair down, secured with a hot pink hairband, contrasting Patricia's ponytail with a pink scrunchie, reflecting her traditionally feminine presentation.

Isabel exhibits a calmer, more studious, and rule-abiding disposition compared to Patricia's rebellious and outspoken nature. She adjusts to the school environment slightly faster, displaying greater empathy towards others and a willingness to follow regulations. Academically inclined, she excels particularly in geography, focusing more on her studies and social integration than her sister. While both twins initially chafed at attending St. Clare's, Isabel demonstrates less overt defiance.

As the quieter, more demure counterpart in their dynamic, Isabel often resists Patricia's schemes to break rules, viewing them as irresponsible, and occasionally voices disapproval of her twin's impulsive actions. Despite this friction, she participates in joint activities like Twin Switches when pressured, such as impersonating Patricia to fulfill chores. Classmates generally perceive Isabel as adorable, pretty, and a "Nice Girl" due to her down-to-earth, empathetic nature and pleasant, often smiling demeanor.

Throughout their time at St. Clare's, Isabel evolves from a secretive and aloof attitude into a more organized and diligent student who consistently adheres to school rules, embodying traits of a "Proper Lady." This development occurs alongside Patricia's, though their personalities remain distinct, with Isabel retaining her quieter, kinder, and more feminine characteristics.