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Reiner is an average-sized boy with short blond hair. He can usually be seen wearing his school uniform when in class.
Reiner is not shown interacting much with students from other classes, but is shown to be very kind to his friends in his homeroom. In particular, he is shown to provide emotional support for his friend Bertolt, and will often aid him in trying to win the heart of their friend Annie.
Shortly after the start of the new school year, there is an annual dodge-ball tournament held and Class Three easily wins the first match. Reiner and his friends get prepared to go up against Class Four; towards the end of the match, Reiner nearly eliminates Eren but the ball is caught and used to eliminate Annie, falling to their fellow freshmen.
All the new students are tasked with enrolling in an after school club on campus; hearing Eren and Jean mention how nice it would be to join an anti Titan club, Reiner arrives with his friends and mentions there once was such a club that existed. Traveling to the older school house, they discover the headquarters of the Survey Club and apply for membership. Reiner, along with the others present, are told by Hange that they are unable to officially enroll to the club due to its secretive nature; as a cover, Reiner decides to also enroll in the school's judo club.
Upon hearing an annual folk dance will be taking place soon, Reiner immediately goes to Krista and invites her as his date. Soon, Eren notices he is in better spirits than usual and tells him about it; just then, Reiner goes to try and catch Krista as she falls but Mike gets there first, causing him to express some jealousy. When the upperclassmen challenge the first year students during the athletic fair (with the loser having to attend the dance with the Titan students), Reiner immediately becomes nervous and is among the first ones to side with Eren.
At the start of the competition, the two sides are evenly matched. However, the upperclassmen begin to take the lead when they tear a hole in a Titan net deployed by Eren. When a third year student named Mike manages to free himself, Reiner squares up with Mike and lunges towards him. Remembering how he saw Krista cheer them on at the start of the competition, Reiner sacrifices himself to cover the hole; Mike commends Reiner for his ingenuity just before he throws him aside, allowing the other upperclassmen to escape. Ultimately, the upperclassmen prevail and the first years are forced to dance with the Titan students.
Upon hearing there is an upcoming Summer Festival, Reiner immediately goes with Bertolt and asks Krista to attend it with him. When Krista agrees and asks if Ymir can join them, Reiner volunteers that she pair up with Bertolt; this is swiftly shot down by Ymir who insists she will be with him and Krista. Reiner later hears that Bertolt asked Annie to accompany him as part of the group; Reiner then pulls out an itinerary list he made years ago and mentions how Bertolt should have told him. When Bertolt begins to second-guess himself, Reiner insists he, Krista and Ymir will be there to support him; he then starts to go over with Bertolt how he can impress Annie and get her to notice him over Eren.
Arriving at the festival, Reiner and Bertolt are stunned seeing how Krista and Annie look; he blurts out how cute Krista is before he can stop himself. After seeing Bertolt begin to falter after Eren, Mikasa and several others join them from Class 4, Reiner instructs him to simply move on to the next step. As the night goes on, Reiner is too enthralled by the sight of Krista that he is initially unaware that Bertolt has disappeared. He helps the others search but Annie dismisses them to go find Bertolt on her own. The next day Reiner, Krista and Ymir applaud Bertolt after he was able to spend some time alone with Annie after all; however, his attitude swiftly changes after Bertolt mentions that they merely went back to the cookie cutting booth he was at when Annie found him without asking her to be his girlfriend. This causes Krista to begin slapping him while Reiner looks on.
- Bertolt Hoover - Reiner's best friend since childhood, he is never seen very far from Bertolt. He also recognizes that Bertolt has a crush on Annie and tries to come up with a plan to help pair the two together.
- Krista Lenz - Another of his classmates, Reiner harbors a massive crush on Krista. At every opportunity or social event, Reiner invites Krista as his date.
- Eren Yeager - While they have not interacted very often, Reiner recognizes that humans are treated inferior to Titan students and respects how Eren always tries to fight against them. After learning about the fabled Survey Club, Reiner immediately informs Eren about it and even goes along to become a member.