Annie Leonhart attends Attack Junior High School as a first-year student, recognizable by her blue eyes and blonde hair typically tied back, with bangs covering her right eye. She wears the standard female uniform: a white shirt, blue skirt, blue bow, white indoor shoes, and socks, often adding a sweater vest. Her attire changes for events: a dark blue tracksuit bearing her name for dodgeball, a cheerleading uniform for Sports Day, and a light violet yukata patterned with green and yellow circles for the summer festival.

Initially distant and reserved, Annie maintains a cold, unemotional demeanor, though she softens in rare moments, particularly around specific individuals. She feels deep embarrassment over seemingly trivial matters, like admitting her fondness for cheeseburgers after Eren Jaeger's public declaration made her a target of ridicule, fueling her initial grudge against him. Performance anxiety plagues her, causing her to freeze on stage during a school talent show and avoid large audiences. Despite her introversion, she confronts others directly when challenged, standing up to Mikasa Ackermann during disputes over Eren or refusing to back down from student council duties.

Her story starts on the first day of school, introducing herself as a transfer from Wall Maria South Elementary School. Her rivalry with Eren escalates during a dodgeball tournament; overhearing his cheeseburger preference, she vows to defeat his team. Though her team dominates initially, they lose when Connie Springer hits her with a ball she forgets to dodge. Eren's apology leads to reconciliation, and she joins him, Mikasa, and Armin Arlert for lunch, sharing her cheeseburger steak after Eren drops his.

As a student council member, Annie investigates unauthorized clubs, specifically targeting Eren's Survey Club for operating without oversight near Wall Rose. She confronts the club and engages in a rock-paper-scissors match to determine its fate. Losing to Eren, she admits her earlier grudge was exaggerated and accepts defeat. Her interactions with Eren grow complex when she attempts to invite him to a school dance. Prevented by Mikasa, Annie fabricates a story about Titan delinquency patrols to separate Eren from her. When Eren declines her invitation due to prior plans with Armin, she initially avoids the dance but later attends the athletic fair at Krista Lenz's urging. After Eren compliments her cheerleading uniform, she agrees to participate but flees upon overhearing his dance plans.

At the summer festival, Annie attends with Bertholdt Hoover, Reiner Braun, Krista, and Ymir, wearing a yukata gifted by her father. She engages in a competitive shooting game rivalry with Mikasa, winning prizes. After Bertholdt protects her from spilled shaved ice, she later finds him crying at a die-cutting booth. Annie confesses she dislikes crowds but feels comfortable around him, comparing him to "air" for his unobtrusive presence. They reconcile and watch fireworks together.

Annie participates in Class Three's bakery stall during the Culture Fair but hides when Eren visits. She is later forced to perform as a replacement singer in a Battle of the Bands. Growing stage fright and audience pressure to confess feelings for Eren threaten to overwhelm her until Bertholdt's off-key singing distracts the crowd.

Her relationships include a developing friendship with Eren after their reconciliation, sparking a rivalry with Mikasa. She befriends the Peering Titan through Titan language classes and collaborates with Hitch Dreyse and Marlo Freudenberg in the student council. Hitch often annoys her, as seen when she and Krista compete for Annie's attention during the Culture Fair. Annie's dynamic with Bertholdt is nuanced; she dismisses his initial festival invitation but later acknowledges his reliability, though she largely ignores him during the event itself. She also punches him after he shows off his physique.