Armin Arlert, a first-year student at Attack Junior High School, has blond hair with straight-cut bangs and blue eyes, usually seen in the school uniform minus the black jacket. His defining trait is the constant use of a futon named Bigbee, featuring a pink and yellow polka-dotted kitten head design, which he wraps around himself for comfort. In warmer weather, he switches to a mesh-structured water-holding futon for cooling. This attachment originates from a traumatic dodgeball tournament failure that caused him to withdraw, spending extended time studying fantasy role-playing games at home.

Shy and timid, especially in social settings, Armin strongly dislikes cold weather, often skipping school during colder months to stay in an overheated room. His childhood friends, Eren Yeager and Mikasa Ackerman, encourage him to engage more with the outside world. Despite initial reluctance, he shows maturity and perceptiveness, offering logical perspectives during group activities. He excels academically, achieving perfect test scores and tutoring classmates like Sasha Braus and Conny Springer.

His story starts in isolation after the dodgeball incident. Eren and Mikasa visit, using food to coax him out, though he remains wrapped in Bigbee. When upperclassman Levi, a member of the unauthorized Scout Regiment club, steals Bigbee for repairs, Armin pursues him to school. After Levi returns the mended futon, Armin joins the Scout Regiment, marking his reintegration. He participates in school sports competitions, cultural festivals, and a "Test of Courage" where he faces supernatural pranks.

Armin becomes increasingly involved in school activities. He designs posters for Eren's student council campaign, though Eren's controversial speech undermines it. During the gourmet competition, he serves as a salesperson for Eren's cheeseburger stand, devising tactics like double-cheese offers. He later helps retrieve stolen food stands from Titans. Exploring creativity, he joins a band for the "Battle of the Bands" as a bass guitarist, proposing dramatic stage effects that get vetoed.

Relationships are central. His bond with Eren and Mikasa is foundational, with Eren offering brotherly support and Mikasa protective companionship. Armin mediates conflicts, like diffusing tensions between Mikasa and Annie Leonhart. His dynamic with Levi shifts from distrust to appreciation after Levi repairs Bigbee. He maintains supportive friendships, tutoring Sasha and offering candid advice to Jean Kirstein about potential pranks.