Jean Kirstein enters Attack Junior High School radiating arrogance and overconfidence. This first-year student considers himself superior to most classmates and obsesses over appearing cool and gaining popularity, especially with female students. He stands slightly above average height, sporting short light brown hair in a two-toned undercut, and wears the school's standard black Japanese middle school uniform.

Jean frequently clashes with Eren Yeager over their opposing ideologies; Jean prioritizes a peaceful school life, while Eren is driven to eliminate Titans. This rivalry intensifies when Jean develops a crush on Mikasa Ackerman, fueling resentment towards Eren due to Mikasa's unwavering loyalty to him. Jean's infatuation with Mikasa often motivates his actions, though she remains indifferent. Academically assigned to Class 4, his close friend Marco Bodt serves as a stabilizing influence, often mediating Jean's conflicts.

During school events, Jean exhibits competitiveness and occasional leadership. As captain of the Class 4 dodgeball team, he struggles with uncooperative teammates like Connie Springer's inept playing and Sasha Blouse's distraction by food. Mikasa's reduced performance due to Eren's absence further frustrates him. Jean attempts to deceive Mikasa by disguising himself as Eren, but the plan fails. His team advances to the finals only to lose to a team of Titans. Later assigned to the Wall Beautification Club, Jean initially skips activities, incurring the wrath of club president Rico Brzenska. Punished with window-cleaning duties, he negotiates with Rico to reduce her lectures in exchange for his regular attendance.

Jean's desire for popularity leads to several humiliations. He receives anonymous love letters, boasting about them to classmates and imagining admirers like Sasha Blouse, Christa Lenz, or Mikasa. The letters are revealed to be from a Titan, orchestrated by Annie Leonhart. Jean tries to politely reject the Titan, resulting in being flattened. During the Athletics Fair, Jean uses unauthorized vertical maneuvering gear during the ball-toss event, drawing Rico's ire. After she criticizes him for shaming the Wall Beautification Club, their argument triggers her rampage, causing their team's loss.

Jean participates in the Cultural Festival's Battle of the Bands, joining Eren's group. He appoints himself lead guitarist and frontman, but creative differences with Eren cause him to quit. He later rejoins upon learning the band would be disqualified without original members. The band performs well but loses to a group led by Levi. Jean assists Eren in sabotaging a rival Titan band with wasabi but is quickly incapacitated.

In the student council presidential race, Jean runs to gain attention. After initial campaign failures, he falsely claims endorsement by the popular band "No Name," boosting his poll numbers. Despite this, he remains behind another candidate, Flegel Reeves, and continues strategizing to win. Jean also experiences quieter moments, such as defending teacher Erwin Smith from Eren's criticisms on a school trip and complaining about melted shaved ice at a summer festival.