Patricia "Pat" O'Sullivan, an identical twin born alongside her sister Isabel in summer 1927, shares a birth timeline with their cousin Alison. Initially head girls together at Redroofs School, Pat fiercely resisted her parents' decision to send them to St. Clare's instead of Ringmere School to prevent them becoming "spoilt and snobbish."

Pat possesses dark brown wavy hair, deep blue eyes, and a merry smile, her pleasant voice carrying an Irish lilt. Generous and friendly, she also exhibits pronounced stubbornness and a hot temper. This volatility fueled her arguments against attending St. Clare's and led her to refuse chores like lighting fires or cleaning boots for older students. Early arrogance earned Pat and Isabel the nickname "stuck-up twins" from classmates.

Her rebellious streak drove her to skip extra French work for the cinema and join pranks against history teacher Miss Kennedy. Notably, when banned from town visits, Pat successfully impersonated Isabel, who had completed the required chores, to sneak out. She later confessed this deception to sports captain Belinda Towers after being selected for the lacrosse team. Belinda, recognizing Pat's natural athletic talent, allowed her to remain on the team.

Pat demonstrates protective instincts, supporting Kathleen Gregory during money theft accusations by explaining the situation to headmistress Miss Theobald alongside Isabel. She comforts Sheila Naylor after arguments and seeks advice from head girl Winifred James. Her loyalty includes sharing punishments, such as confessing with others to Janet Robins' firecracker prank, resulting in communal consequences like chimney sweeping.

Significant character development occurs across terms. Initially dismissive of St. Clare's, Pat grows to appreciate the school, surprising herself by not wanting to leave. By the second book, she shows greater responsibility, helping prepare cousin Alison for school life despite Alison's "feather-headed" tendencies. Pat mediates conflicts, advising Alison against vanity and tears, though she struggles with patience. Her role expands in addressing injustices, particularly mistreatment of younger students.

Her leadership sometimes falters under bias. She spearheaded the first form's exclusion of Margery Fenworthy after outbursts, instructing classmates not to cheer Margery's lacrosse goals—a decision later regretted. Pat also wrongly accused Margery of vandalizing her knitting and nature project, unaware Erica framed her. However, Pat accepted Margery's heroism during a sickroom fire and supported her reconciliation with her father.

Relationships define her journey. Her central twin bond with Isabel enables their early identity switches, though Pat often dominates decisions. She forges lasting friendships with prankster Janet Robins and later Bobby Ellis, bonding over shared mischief. As the series progresses, her prominence diminishes slightly, shifting toward stabilizing background roles while newer characters take focus. By the upper forms, she evolves into a responsible figure, eventually becoming joint head girl with Isabel.

Academically, Pat faces challenges in French, clashing with teacher Mam'zelle and requiring remedial work. Her strengths emerge in sports, particularly lacrosse, where her agility and fierce competitive determination shine, mirroring her personal growth from rebelliousness to reliability.