Ereri

Ereri (エレリ) is a slash ship between Eren Yeager (also spelled as Eren Jaeger in the FUNimation dub of the anime) and Levi Ackerman (often incorrectly romanized as Rivaille) from the Attack on Titan series. Levi consistently dominates fan popularity polls in both Japan and the West, and Eren is the series' main protagonist.

Because of this, it has become the most popular slash pairing of the franchise and is also widely considered to be one of the most popular yaoi ships in Japan (particularly when it comes to doujinshi sales). The two are also represented in more official art and merchandise than any other Attack on Titan pairing because of the relative popularity of the characters.

Fanon
Some fans of Eren Yeager and Levi Ackerman feel like the two would be a beautiful couple, that they would develop a romantic/sexual relationship overtime. The facts that Levi and Eren forgive each other for what they have done to each other, such as the court beating (Eren doesn't hate him for it and forgives him).