EruRi

EruRi is the slash ship between Captain Levi Ackerman and Commander Erwin from the Attack on Titan fandom.

Canon
In "A Choice with No Regrets" it is shown that Levi once intended to kill Erwin to gain citizenship on the surface. However, Levi later chose to instead follow Erwin and work with him to serve humanity.

Overtime Erwin earned Levi's trust and subservience. Despite this Levi still spoke rudely around him sometimes, though Erwin didn't seem to mind. Erwin was also one of the few people who could maintain a casual conversation with Levi which enabled them to converse with one another on equal grounds.

When Erwin asked Levi if he would take the titan serum Kenny had swiped, Levi requested that Erwin just give him the order. During the Survey Corps' final meeting before the Battle to retake Wall Maria, Levi closed the door behind the other members to talk to Erwin privately. Levi made it clear that he was concerned about Erwin’s physical condition and told him to stay put. When Erwin declined, Levi threatened to break his legs as a joke. However, Levi soon realised that he couldn't change Erwin's mind which left him looking slightly glum.

Facing imminent defeat against the beast titan, Erwin began to grieve over the failure of his attempts to learn the truth of the world. He thought back to the many soldiers who had died for the sake of humanity and who he had used to get closer to his dream, picturing himself standing on a mountain of their corpses. Erwin began to question his dream as a childish delusion. Levi knelt down and told Erwin that the Survey Corps wouldn’t have been able to get so far without him. He then told Erwin to give up on his dreams, die and lead the new recruits into battle, promising to take down the beast titan. Despite his dreams coming to a bitter end Erwin gave a smile of relief at Levi's words and soon charged into battle. When Levi failed to kill the beast titan and became the target of multiple mindless titans, Levi recalled his promise to Erwin and looked pained by his failure.

Levi changed his mind on saving Armin with the serum when a soldier delivered a critically wounded Erwin to him, causing Eren and Mikasa to lash out. Once they had been restrained Levi proceeded to inject Erwin. At this point Levi thought back to Erwin's dreams and the last words of Kenny. Erwin, in his near-death state, suddenly knocked the needle out of his hand. Levi then recalled the look of relief on Erwin's face and this led to him choosing to let Erwin die peacefully, despite wanting to have him back. Levi told Erwin that his promise to defeat the beast titan would have to wait, but Erwin had already passed.

In an interview with Isayama, he revealed that Erwin's bones were recovered some time after his death and Levi was the one who buried him.

Fanon
Many fans have noticed that the relationship between Erwin and Levi is unique, based on deep trust and respect. Also, they have a longer history together so they have a strong personal bond. Levi did always follow Erwin around, especially in the anime (like in season 1, episode 22) and after Erwin’s death he did claim that it was his personal feelings which led to his decision.

In one of Isayama's interviews he was asked about Levi's preferred type of person and answered: "A type of woman he likes? Since he’s short, small people tends to like tall people… I guess he might like tall people.” Since Erwin was one of the tallest in the series it is probable that Levi took a liking toward Erwin.

Many shippers have created comics and arts to show their support of the ship.