Patrίcia O'Sullivan, known as Pat, hails from privilege and previously attended the elite Redroofs school. There, she excelled academically and athletically, serving as head girl and sports captain. Her parents enrolled her at St. Clare's to counter her developing snobbishness, a decision she initially chafed against.

Pat sports dark brown hair, often pulled back in a tomboyish ponytail secured with a pink scrunchie – a deliberate contrast to her identical twin Isabel's style of wearing her hair down with a headband. Both share distinctive dark blue eyes.

Pat bursts with a hot-headed, rebellious, and defiant personality, frequently challenging authority and school rules. She exerts leadership over Isabel, pressuring her into schemes like twin switches to evade chores or punishments. Her initial arrogance and dismissive behavior toward classmates earn her the nickname "the stuck-up twin."

A skilled lacrosse player, Pat's athletic prowess catches the eye of sports captain Belinda Towers, who selects her for the team despite behavioral issues. This opportunity proves pivotal when Pat confesses to deceiving Belinda during a twin switch, revealing her developing sense of integrity.

Throughout her time at St. Clare's, Pat undergoes significant change. Early conflicts include balking at menial tasks for seniors and clashing with teachers like Mam'zelle over academic expectations. She gradually integrates, showing protectiveness by championing vulnerable peers like her cousin Allison against bullying. Her relationship with Isabel shifts from dominance toward mutual respect, though rivalries occasionally resurface.

In later narrative arcs, Pat's role recedes as newer characters emerge, though she remains a background stabilizing presence. By the series' conclusion, she achieves the position of joint head girl alongside Isabel, signifying her acceptance of responsibility and institutional values.