Hanni O'Sullivan, originally named Patricia "Pat" O'Sullivan in the source material, is one of a pair of identical twin sisters central to the narrative. Sent to St. Clare's boarding school by her parents to correct her spoiled and arrogant behavior, alongside her sister Isabel (adapted as Nanni), Hanni initially resents the school. She views it as inferior to more elite institutions and deliberately engages in rebellious acts to secure expulsion.

Hanni possesses a dominant, strong-willed personality compared to her twin, often leading their mischief. This includes organizing pranks against teachers, like tricking the French mistress Mam'zelle, and participating in forbidden activities such as midnight feasts. Her early defiance manifests in refusing duties expected of younger students, like serving older pupils. She exploits her identical appearance to Isabel for subterfuge, notably pretending to be her sister to bypass restrictions.

Her character development focuses on gradual reform. Exposure to St. Clare's ethos and interactions with peers, including head girl Winifred James and games captain Belinda Towers, diminish her snobbishness. Key turning points involve witnessing the consequences of her actions on others and recognizing the school community's value. By her first term's end, Hanni accepts St. Clare's and commits to personal growth, shedding much initial arrogance.

In subsequent terms, Hanni evolves into a stabilizing presence. She demonstrates loyalty, particularly supporting mistreated peers like Margery Fenworthy, whom she initially misjudges. She displays leadership in sports like lacrosse (adapted as handball regionally) and academic responsibility, eventually rising to become joint head girl with her sister. Her close relationship with Isabel endures, though Hanni's assertiveness typically positions her as the primary decision-maker.