Patricia O'Sullivan, called Pat, comes from a family worried about her becoming snobbish, prompting her enrollment at St. Clare's with her identical twin, Isabel. Initially resistant, she rebelled openly, expressing disdain for the school and its rules while urging Isabel to help evade authority. Pat is hot-headed, stubborn, and prone to rule-breaking, often talking back to adults and drawing her sister into schemes.

She has dark brown wavy hair, usually in a tomboyish ponytail held by a pink scrunchie, deep blue eyes, and an energetic presence. Her identical looks to Isabel enable twin switches, like impersonating Isabel to skip chores and visit town—a deception she later confessed to due to guilt.

Pat's development begins gradually. An unexpected kindness after being chosen for the lacrosse team despite prior defiance prompts her to volunteer for tasks she once resisted. She demonstrates emerging integrity by taking shared blame for pranks targeting teachers and shows compassion by intervening against bullying, such as defending Doris. Her protective instincts extend to animals, actively helping rescue and care for an injured stray puppy found near the school.

Throughout her journey, Pat maintains a leadership tendency within the twin dynamic, often initiating plans while Isabel follows. Despite mellowing, traces of her spirited nature persist, including joining an unauthorized circus trip that brings punishment. Her peer relationships evolve from initial friction to mutual respect, especially with previously disliked classmates like Janet, fostered by shared experiences and collective problem-solving.

Her family relationships also shift, moving from conflict over her school placement to eventual acceptance and appreciation of St. Clare's.