Armin Arlert, a timid first-year student at Attack Junior High, navigates school life with intellectual sharpness and the constant companionship of Bigbee—a vibrant futon adorned with pink and yellow polka-dotted kitten heads. This unconventional security blanket, draped over his head or shoulders, anchors his well-being; its absence triggers physical malaise, starkly evident when Levi’s temporary confiscation for repairs leaves Armin visibly shaken.

Haunted by a humiliating dodgeball failure in elementary school, Armin retreated into solitude, spending a summer sequestered in his room immersed in role-playing games. Within these fantasy realms, he reimagined Bigbee as a mystical cloak, its whimsical design fueling his vivid imagination. Reluctant to rejoin society, he only ventured back after persistent encouragement from friends Eren and Mikasa, who coaxed him into a mandatory dodgeball event—a tentative step toward rebuilding social connections.

Academically gifted, Armin tutors peers like Sasha Blouse in mathematics, yet social interactions outside his tight-knit circle unsettle him. His tactical mind shines during school challenges, whether orchestrating a plan to disguise Jean as Eren to boost Mikasa’s morale or devising promotional strategies for a festival cheeseburger stand. Observant and empathetic, he diffuses tensions between Mikasa and Annie Leonhart, discerning Mikasa’s reliance on Eren and Annie’s covert love for cheeseburgers.

Initially intimidated by Levi, Armin gains respect for the older student after Levi repairs Bigbee, uncovering unexpected kindness beneath his austere demeanor. Armin’s involvement in clubs—the Survey Club, Wall Beautification Club, and a Test of Courage—highlights his cautious yet collaborative spirit. He even proposes a hard rock theme for Eren’s band during a Battle of the Bands, though Mikasa swiftly overrules the idea.

Prone to winter absences due to cold sensitivity, Armin retreats to overheated rooms, yet adapts creatively by designing a breathable mesh futon for summer. His gradual growth surfaces during a daring mission to reclaim Bigbee from Levi, where he mobilizes classmates, revealing nascent leadership beneath his reserved exterior.