Jean Kirstein is a first-year student in Class 1-04 at Attack Junior High School, sporting an undercut hairstyle with two-toned coloring and the standard uniform. His primary motivation centers on gaining popularity, especially with female classmates, and he perceives himself as intellectually superior to most peers. This self-view fuels frequent conflicts, particularly with rival Eren Yeager. Jean develops an immediate romantic interest in Mikasa Ackerman, intensifying his jealousy toward Eren and leading to repeated confrontations. His close friendship with Marco Bodt provides stability, as Marco often mediates conflicts and offers support.

As captain of his class dodgeball team, Jean's competitive nature surfaces through frustration over teammates' unconventional behavior like Sasha Blouse's preoccupation with food and Connie Springer's poor athletic performance. He becomes dismayed when Mikasa's abilities decline due to Eren's absence, prompting him to disguise himself as Eren in a failed attempt to motivate her. Eren's timely arrival allows the team to advance to the finals, where they face a team of Titans and suffer defeat.

Jean initially resists school clubs, particularly skipping activities for the Wall Beautification Club led by second-year student Rico Brzenska. Confronted about absenteeism, he receives punishment assignments like window cleaning. Rico's introduction of the club's omni-directional mobility gear sparks Jean's interest as he envisions using it to enhance his popularity. He competes with Eren over the gear during cleaning tasks, resulting in further disciplinary actions. Witnessing Titans vandalizing Wall Rose during a club trip leads Jean to understand the club's true purpose and align more closely with Eren's perspective on Titan threats.

Jean experiences social embarrassment after discovering a love letter in his locker; he boasts to classmates, assuming a female admirer sent it. He entertains fantasies about female classmates like Mikasa and Annie Leonhart being interested in him. When Annie approaches him privately, he misinterprets her intentions, learning she wrote the letter on behalf of a Titan that developed affection for him after he once fed it his bento. Jean's attempts to politely reject the Titan result in physical retaliation from both the Titan and Annie. Sasha later consoles him by using food items to simulate romantic attention.

For the school's Athletics Fair ball-tossing event, Jean devises a strategy involving unauthorized use of the Wall Beautification Club's mobility gear. Rico intercepts his plan, pelts him with balls, and leads her team to physically punish him. His team loses the competition, resulting in mandatory participation in the folk dance with Titans.

Jean joins Eren's band for the Cultural Festival's Battle of the Bands, appointing himself as lead guitarist and frontman. This self-promotion causes friction until Mikasa negotiates shared leadership between Jean and Eren. Creative disagreements lead Jean to quit, though he later rejoins upon learning original members must participate to avoid disqualification. The band performs successfully but loses to upperclassmen.

He runs for student council president believing the position will boost his popularity. After initial campaign struggles, he falsely claims endorsement from the popular band "No Name" to improve polling numbers. His efforts are undermined when opponent Oruo Bozado covers all bulletin boards with his own posters. Jean and Eren respond by placing their posters over Oruo's, but Jean ultimately receives zero percent in opinion polls.

Throughout the year, Jean participates in various activities: summer festivals arguing over melted shaved ice, test-of-courage events where upperclassmen prank students, and weekend trips with the unauthorized Scout Regiment club. His interactions with faculty like advisor Erwin Smith and second-year students like Levi demonstrate ongoing junior-senior tensions, often resulting in disciplinary consequences for Jean and his classmates.