EreRei

EreRei is the slash ship between Eren Jaeger and Reiner Braun from the Attack on Titan fandom.

Canon
The two had a pretty close relationship with each other before Reiner and Bertholdt revealed to be the Armored and the Colossal Titan. Eren looked up to Reiner and saw him as a role model and an older brother. The two trained together and Reiner taught Eren about the importance of the duties he had as a soldier and that he had to protect the weak. It was revealed that Reiner was the one Eren looked up to and respected the most. After Conny and Jean can't any suggestions to help Eren with his training, he visits Reiner Braun and Bertholdt Hoover for help. Although they aren’t too close at first, the group bonds over their similar experiences during the fall of Wall Maria. The three exchange tips and start to become close friends.

When Eren is on top the Wall recovering from the ordeal at Utgard. He questions about Ymir's condition and gives Reiner a hand up the Wall as Historia explains her true parentage. Helping Armin up as well, Eren goes to check on Reiner after hearing about how bad his arm was injured in the Castle; this, along with the moment that Reiner was almost killed by the Female Titan, causes Reiner to comment that he doesn’t expect that he'll live much longer and that they should keep fighting until the breach in Wall Maria is covered. After Bertholdt tells Reiner that they won't have to wait much longer and can return home soon, Hannes arrives with news. As Hange orders them to stay on standby in Trost District, Reiner goes to speak with Eren in private. Reiner reveals himself as the Armored and reveals Bertholdt as the Colossal Titan. He then respectively tells Eren that they won’t attack the Walls if Eren returns to their homeland with them. Eren recalls Hange theory about how Reiner and Bertholdt might be accomplices to Annie and tries to persuade Reiner that he's just tired and not thinking straight. In response to Eren's disbelief, Reiner removes the bandage on his arm and reveals that the wounds he had were regenerating. Mikasa attempts to protect Eren but Reiner and Bertholdt transform into their Titan forms and attack. Reiner grabs Eren while Bertholdt kidnaps Ymir and Eren, disgusted by his former friends, transforms himself.

Eren later started to despise Reiner after he revealed himself as the Armored Titan. He started to only view Reiner as a despicable traitor to humanity you had no right at all to live. Eren wanted to make sure that he killed both Reiner and Bertholdt with the most slowest gruesome pain they would ever feel. He felt that that would be the only death suited for disgusting traitors like them.

When Eren and Reiner meet again four years later, Eren's hatred towards Reiner seems to have gone down to some point, expressing to Reiner that it is good that he had managed to return to his hometown. In return, Reiner was shocked by Eren's return to him and could barely say a word to him. Eren expresses sympathy towards Reiner after learning the truth about Reiner's mission within the Walls and the world's hatred towards Eldians and experiencing the same infiltration mission within Marley that Reiner carried out years before. Eren admits he and Reiner are the same, driven by freedom for their people and manipulated by corrupt leaders for the actions of their ancestors. Eren even admits his own anger towards Reiner and Bertolt was a bit too much due to him not knowing their reasons why.

Fanon
On AO3, the ship has 246 fanfics. Fans who view their relationship as platonic usually put Reiner in a 'big brother' role. Many fans like to point out how Eren and Reiner's characters are foils of each other, especially during the 4th season. A Twitter artist popularized a modern interpretation of their relationship, where in Eren is portrayed as a "toxic ex" and Reiner's emotionally sensitive side is exaggerated.

Fandom
FAN FICTION
 * Eren/Reiner tag on FanFiction.net
 * Eren/Reiner tag on FanFiction.net