Armin Arlelt, a first-year student at Attack Junior High School, shares a deep childhood bond with Eren Jaeger and Mikasa Ackerman. His personality is markedly timid and reserved, stemming from a crisis of self-confidence triggered by an elementary school dodgeball tournament loss he blamed on himself. This incident caused him to withdraw completely, avoiding school and retreating to his room for extended periods. During this isolation, he immersed himself in studying role-playing games and fantasy worlds, cultivating a vivid imagination that transformed his futon, Bigbee, into a magical cloak central to his identity.

Physically, Armin possesses blond hair cut straight across his bangs and large blue eyes. His defining characteristic is the constant presence of Bigbee, a pink and yellow polka-dotted futon lined with white fur. He wears it draped over his body or wrapped around his head like a hood, insisting it is part of him and claiming its absence makes him physically ill. In warmer weather, he switches to a mesh-structured futon designed to hold water and keep him cool. He dreads cold weather, often skipping school during colder seasons and staying in an overheated room that discomforts others.

Academically, Armin excels, achieving top scores like 100 on exams. He willingly tutors classmates such as Sasha Braus in subjects like math. Despite his shyness, he demonstrates perceptiveness, noticing shifts in Mikasa's mood when Eren is absent and devising plans to cheer her, including disguising Jean Kirschtein as Eren during a dodgeball match. His loyalty to Eren and Mikasa is evident; he relies on their support while also providing emotional stability in return.

His journey begins when Eren and Mikasa convince him to leave isolation to participate in a mandatory dodgeball tournament. Though initially distrustful, he agrees and returns to school wrapped in Bigbee. During the tournament, he observes Mikasa's diminished performance due to Eren's absence and orchestrates Jean's disguise attempt. Later, Levi forcibly takes Bigbee to repair a tear, prompting Armin to pursue him with Eren and Mikasa's help. After Levi returns the mended futon, Armin joins the Attack Junior High Scout Regiment, viewing Levi more favorably.

Armin participates in various school activities: cleaning windows with the Wall Beautification Club under Rico's orders, competing in Sports Day events, and cowering behind Mikasa during a "Test of Courage" event when encountering staged school "wonders" like the Laughing Mona Lisa. He aids Eren's campaign for student council president by designing posters, though Eren's controversial speech backfires. For the school festival, Armin serves as a salesperson at Eren's cheeseburger stand, promoting limited-time deals, and joins Eren's band as a bass guitarist for the Battle of the Bands. He suggests a dramatic rock finale involving setting a guitar on fire—an idea Mikasa rejects. The band loses to upperclassmen Hange and Levi's group.

His relationships include an implicit trust in Eren, choosing him as a dance partner due to social anxiety. Mikasa remains another steadfast friend whose emotional state he monitors closely. He maintains a supportive dynamic with Sasha, tutoring her despite occasional frustration with her distractions. Initially intimidated by Levi, Armin's view evolves into gratitude after the futon incident.