Alison O'Sullivan is the cousin of main characters Pat and Isabel O'Sullivan. She possesses auburn hair and blue eyes, setting her apart from her dark-haired cousins, though she shares a family resemblance, particularly in eye color. Alison is notably clumsy and accident-prone; a significant early incident involves her burning important papers belonging to an older student. Her behavior includes pronounced hero-worship, specifically directed at a dominant student named Madeleine. Alison admires Madeleine and constantly seeks her approval, often displaying fawning or subservient behavior.

She is characterized as gullible and academically challenged. For example, she readily adopts an unhealthy diet scheme promoted by another student. Her academic struggles result in poor grades and contribute to social missteps. Alison also struggles with sports despite wanting to participate. This clumsiness extends beyond academics and sports, causing difficulties in following school rules and integrating smoothly with her peers.

Compared to Pat and Isabel, Alison displays more overtly childish and less mature traits. Her social standing suffers due to her behavior, resulting in unpopularity among other students.