Annie Leonhart, a first-year student at Attack Junior High School, possesses blue eyes and blonde hair typically tied back, her bangs covering her right eye. She maintains a consistently unemotional expression regardless of her mood. Her standard school uniform includes a white shirt, blue skirt, blue bow, and indoor white shoes with socks, often layered with a sweater vest. Event-specific attire includes a dark blue tracksuit with her name for dodgeball, a cheerleading uniform for Sports Day, and a light violet yukata decorated with green and yellow circles for the summer festival.

Reserved and introverted, Annie initially appears distant and unapproachable, struggling with social interactions. She experiences deep embarrassment over seemingly trivial matters, like her fondness for cheeseburgers after Eren Jaeger's public declaration made it awkward for her to admit. This led to a temporary grudge against him. Though outwardly cold, she gradually softens, especially toward Eren after he apologizes, leading her to share meals with him, Mikasa Ackermann, and Armin Arlert. Annie exhibits performance anxiety, avoiding large crowds and public displays; being forced to sing during a Battle of the Bands event mortified her. Emotionally charged situations easily fluster her, such as attempting to ask Eren to the school dance or reacting defensively when her interactions with him are scrutinized. She demonstrates self-awareness, acknowledging overreactions and apologizing when necessary, and shows kindness, like offering Eren part of her lunch after he dropped his.

Her story starts on the first day of school, revealing she attended Wall Maria South Elementary. During the opening ceremony, Eren’s cheeseburger declaration causes her embarrassment, fueling her determination to defeat him in the dodgeball tournament. In the tournament, her competitive streak emerges as her team faces Eren's class; she nearly eliminates Mikasa but loses when Conny Springer throws a ball she forgets to dodge. Reconciliation follows Eren's apology, and she joins his lunch group.

Serving on the student council alongside Hitch Dreyse and Marlo Freudenberg, Annie actively seeks to disband the unauthorized Survey Club, confronting Eren and his friends at Wall Rose. A rock-paper-scissors game decides the club's fate, culminating in Annie facing Eren. After a prolonged tie, she admits her cheeseburger-related embarrassment and concedes defeat. Later, she attempts to ask Eren to the school dance but is obstructed by Mikasa. Annie fabricates a story about Titan delinquency monitoring to secure time alone with Eren, though her plans are repeatedly interrupted. When she finally asks him, he declines due to prior plans with Armin, disappointing her. She initially avoids Sports Day but is persuaded by Krista Lenz to cheer for underclassmen. Complimented by Eren on her cheerleading outfit, she considers participating until reminded of his dance plans with Armin, causing her to retreat.

At the summer festival, Bertholdt Hoover invites Annie, who reluctantly accepts. She wears a yukata gifted by her father, participating in activities like a shooting game where she develops a rivalry with Mikasa. When a shaved ice cup nearly hits her, Bertholdt protects her and gets stained. After he disappears, Annie finds him at a die-cutting booth, revealing she remembered his hobby. She admits discomfort in crowds but acknowledges feeling at ease with him, comparing his unobtrusive presence to "air." They reconcile and participate in die-cutting together while watching fireworks.

Annie is also skilled in Titan language, befriending the Peering Titan. She assists the Titan in conveying feelings to Jean Kirschtein through letters, though Jean rejects the advances. Later, during a mission to recover stolen chee-burgs from the Titans’ building, Annie and her Titan friend rescue Jean from a lunging Titan, enabling Eren to retrieve a key.