Annie is one of the two abandoned infants left at the doorstep of Pony's Home orphanage on the same snowy day as Candy, making them inseparable childhood friends from the very beginning of their lives. Her background is shrouded in the mystery of her birth, as she was found with nothing but a note from her birth mother explaining an inability to care for her and stating her name. In stark contrast to Candy's boisterous and tomboyish nature, Annie is characterized by her well-mannered, polite, and extremely shy demeanor. She is a docile and quiet girl who often appears reserved and apprehensive, lacking the outgoing confidence that defines her friend.

Annie's primary motivation throughout the early part of the story is a deep-seated desire for security, family, and social acceptance, which stems from her longing for a traditional home with parents. This desire manifests in moments of envy and selfishness, as she occasionally becomes jealous of the attention Candy receives, whether from the orphanage staff or from others. When the wealthy Brighton family arrives at Pony's Home with the intention of adopting, Candy deliberately sabotages her own chance to be adopted so she can stay with Annie. However, when Mrs. Brighton is impressed by Annie's quiet politeness and offers to adopt her instead, Annie accepts without hesitation, prioritizing her dream of having a family over her promise to remain with Candy forever.

Annie's role in the story is primarily that of Candy's first and most emotionally complex friend. Her departure from the orphanage marks a significant turning point, forcing Candy to confront a world beyond their childhood bond. After moving to the Brighton estate, Annie is forbidden by her new mother from sending letters, as Mrs. Brighton wishes to hide Annie's past as an orphan to maintain social standing. In her final letter to Candy, Annie bids farewell, explaining that to become a proper lady, she must hide her origins. This decision to sever ties creates a painful rift between the two.

Their relationship becomes further strained when they reunite at Saint Paul's Academy in London. Initially, Annie pretends not to know Candy, desperate to conceal her past from the other students. Her jealousy resurfaces when she notices the kind and handsome Archibald Cornwell's clear admiration for Candy. Motivated by her romantic feelings for Archibald, Annie begs Candy not to steal him away, accusing Candy of always being everyone's favorite, even back at Pony's Home. This outburst is overheard by the malicious Eliza and Neil, who then publicly reveal Annie's orphan origins, causing her great social anguish. It is during this crisis that Annie and Candy begin to reconcile. While hiding in a cave, heartbroken and feverish, Annie finally accepts that while Archibald cares for Candy, his true feelings are for her, and she and Candy make peace, solidifying their bond once more. Later, when Candy and Annie are forced to leave the Ardley mansion due to the cruelty of the Leagan family, they sleep in a treehouse and declare themselves sisters, cementing their relationship as a family of their own making.

Throughout the story, Annie shows notable development, transforming from a girl who is ashamed of her past and willing to abandon her friendship for social status into someone who embraces her identity and her connection to Candy. While she begins as a timid and insecure child, she grows into a more patient and resilient young woman, learning to wait for Archibald's love and to stand by her friends during difficult times, including the tragedy of World War I and the death of Alistear. Annie does not possess any notable physical or combat abilities; her strengths lie in her quiet politeness and social grace, which initially impress Mrs. Brighton, but her most significant growth is emotional, learning to overcome her envy and insecurity to become a loyal and supportive figure in Candy's life.