Brawny and blond, Reiner's physique is exaggerated to the point of having a well built chest and defined arm muscles. He wears the standard uniform of the school.
He has a tough physique and can be stubborn. He gave his heart to Christa but is always chased away by Ymir. He is seen hanging out with Bertholdt, often giving him helpful advice about being more confident or speaking out. In the class's eyes, Reiner is often admired for his assistance and supportiveness.
Reiner attends his first day of school. He introduces himself to the class and is called out to the courtyard for the entrance ceremony.
Reiner finds his friends complaining about their clubs and he pitches in. He advocates that they are going to have to live with it, as he is facing a similar situation. He follows his friends to locate the Attack Junior High Scout Regiment, where he meets odd upperclassmen who welcome him into the club, despite being in a different one already.
Reiner participates on Annie's team in the game of dodgeball. With his teamwork along with the members, the team manages to win their first match easily. When the ball hits Christa, he rushes for her safety. So happen, Ymir is on guard as she trips Reiner and catches the ball, gaining the praise from Christa in her own hands. On lunchtime, he along with his teammates, offer their Mozuku Seaweed lunch to Annie. Later on the continuity of the match between Mikasa, Annie and Eren, he bets on Annie with some snacks.
Arriving late at the Scouts club, the upperclassmen punish Reiner and the others by doing cleaning duties. As Christa climbs a ladder, she tumbles and begins to fall. Reiner stands behind her ready to catch her, until Ymir intercepts and kicks him in the face, catching Christa instead. After Levi gives the group a challenge, Reiner participates in the school's Sports Day. Christa arrives, having changed into her cheerleading uniform, smiting Reiner. In the last game, shoulder wars, he teams up with Eren, Jean, and Bertholdt against the upperclassmen. When his team charges at the Year 3 leader, Levi, he hits and sends them all flying after taking all of their bandanas.
The upperclassmen plan a sudden "Test of Courage" event at night. Reiner is assigned with Bertholdt, Ymir, Christa, and Sasha. While the girls are in the bathroom, he and Bertholdt wait outside. They see the back of a ghostly figure at the bottom of the stairs, and go to investigate. Reiner sees that there are suddenly loads more staircases leading up and down the school. Reiner and Bertholdt have been climbing up the many staircases they saw and Bertholdt asks if Reiner sensed anything weird, but Reiner tells him it is just his imagination. Bertholdt continues to say that they have been climbing for at least half an hour, yet Reiner again states it is just his imagination. Suddenly they see a huge cannon rolling down from the stairs and it follows them to the gym where everyone else is there running around from the cannon. It is later revealed that the event is a prank played by the upperclassmen.
Reiner asks Christa if she would like to go to the summer festival with him and Bertholdt. Christa quickly agrees, much to his surprise. He then urges Bertholdt to invite Annie, seeing how he always stares at her. Teaming up with Ymir, Reiner formulates a plan in order for Bertholdt ask Annie out at the festival. There, Reiner stands outside with Bertholdt, and the former explains his plan. Operation 1; all Bertholdt has to do is compliment Annie by calling her cute. Ymir, Christa, and Annie then arrive. As Reiner tries to tell how cute Christa looks in her kimono, Ymir kicks him aside and compliments Christa herself. Reiner tells Bertholdt to say something to Annie, and he compliments a knot on her geta strap, to Reiner's disappointment.
Reiner continues with his plan. Operation 2; Bertholdt showing Annie his manly side by playing a shooting game. However, Reiner is shocked when rivalry sparks between Mikasa and Annie and they win every prize in the shooting game stall. Operation 3; Bertholdt protects Annie from staining her kimono. When the wrong people go to buy the shaved ice, Ymir still sneaks behind Jean and pushes the cup towards Annie, so Bertholdt could shield her. Reiner and the others later hide behind bushes and eavesdrop on Bertholdt and Annie. When Bertholdt asks Annie something and she agrees, Reiner congratulates him, only to discover later that he merely asked her to do die-cutting.
Following the election for a new student council president, Reiner and the other candidates start chasing after a notebook that allegedly contains info which could help them win the election. However, after a long pursuit, they discover that it merely contains cleaning tips.
When it is Reiner's turn for his final presidential speech, he says he will be holding training camps and swimming meets to increase the opportunities of physical contact between the students, while a projection screen showcases female students in swimsuits, and all the girls boo at him, while the guys cheer in excitement. Looking at the crowd, Reiner sees Christa looking back at him in disgust, remarking how low his plans are. He is shocked to the point Bertholdt has to drag him off stage.
In the school's 104th festival, Reiner and Bertholdt visit the maid café room, with Christa serving them omelet rice. Smitten by how Christa looks in a maid getup, Reiner thinks to himself that he wants to marry her. Seeing the look on Reiner's face, Ymir spoils his meal, to his shock. Bertholdt offers to switch his omelet with Reiner's, but he refuses and says he had promised Christa to eat it all and tries to wolf it down, with his face turning all colors. He asks for another one, and Christa obliges.
Ymir appears and again tries to spoil his meal, but Reiner shields it this time, telling Bertholdt that he will definitely survive and eat it. Christa later stops Ymir, and when Reiner tries the omelet, he spits fire and falls to the ground with his mouth burned. Bertholdt realizes the omelet's insides contain hot sauce, and Ymir smirks while holding the sauce bottle.
When the teachers get in the way of Eren trying to head to the Titans' building to retrieve his cheeseburgers, Reiner and the others stand up for him and come between them. He later watches Ilse Langnar's broadcast of Eren and the others venturing into the principal's office, and cheers once Eren manages to retrieve a chee-burg.
- Bertholdt Hoover - Reiner believes that Bertholdt should not be afraid of trying to stand out. He shows this by trying to make Bertholdt more well-known among his peers or trying to make him comfortable with talking to others.
- Christa Lenz - Reiner has an unrequited crush on Christa. He is easily charmed by her acts of kindness and is nervous around her, however it appears that Christa does not pay much attention to it.
- Ymir - Ymir is the stopper between Reiner and Christa. She disapproves of Reiner's crush on her and tries to block any interaction between them.
- Hajime Isayama has stated in recent interviews that Reiner is his favorite character in the series.
- When designing Reiner, Isayama took the eyes and eyebrows of David Beckham, as well as some characteristics from a personal friend of his, and added them to the appearance of Private First Class James Francis Ryan (Matt Damon) from Saving Private Ryan.
- In German, the word "Braun" means "Brown."