Identical twin Isabel O'Sullivan, known as "Nanni" in the German dub alongside her sister Patricia ("Hanni"), arrives at St. Clare's boarding school. Isabel possesses dark brown hair typically worn down, secured with a pink hairband, contrasting with Patricia's ponytail, and has deep blue eyes, favoring a feminine presentation.

Initially resistant to attending St. Clare's, she adapts faster than her sister, shifting from rebellion to embracing school life. She develops into a responsible, rule-abiding student with strong organizational skills and notable empathy towards peers and authority figures. While participating in early twin switches to avoid chores, she adopts a more conscientious attitude. Her compassion surfaces when supporting mistreated classmates or teachers under duress, like Miss Kennedy facing personal struggles.

Academically inclined, Isabel excels in geography and prioritizes studies, participating in lacrosse but with less proficiency than Patricia. Her relationships define her: loyal to Patricia despite frequent clashes over rule-breaking, she often bridges differences during peer conflicts. She shows affection for animals, such as the stray dog Blinky. Her interactions with authority, particularly French teacher Mam'zelle, evolve from antagonism to mutual respect following reconciliations.

Positioned as the quieter, more observant twin, Isabel internalizes conflicts but asserts herself at critical moments, confronting Patricia about irresponsible behavior or advocating for fairness during group punishments.