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.

Canon
Although no romantic undertones are evident in either the manga or anime, and their relationship skirts around the term 'friendship', there is a firm sense of mutual respect between Eren and Levi built up over the course of the manga and the two clearly care for each other's wellbeing. On Eren's side, Captain Levi of the Survey Corps represents everything he's come to admire and respect: freedom from the claustrophobic outer walls, hatred of titans and a desire to avoid the fate set out for him (when Eren enquires about Levi's past, Petra informs him that he was previously a thug in the underground city).

The start of their relationship in canon is thus dominated by Eren's hero-worship, and Levi's belief that Eren is little more than a monster with the potential to become an asset to humanity's cause. After being taken into custody, Levi shortly asks Eren "what is it you want to do?" Levi describes Eren's fierce response, that he wants to "join the survey corps and slaughter some titans", as "not bad". Eren's attitude towards titans interests Levi enough that he decides to take responsibility for him, vowing that if he "betrays us or goes beserk, I'll kill him on the spot." When Eren's situation goes south at the trial and it becomes clear that he will be sentenced to execution, Levi makes a show of kicking and degrading him in front of the courtroom in order to demonstrate to the judge that he is capable of keeping him under control. Erwin considers Eren's abilities to be a vital asset, and it is his words along with Levi's brutal display that ultimately seal Eren's fate as a member of the survey corps.

Unlike Eren, who is expressive and hot-headed, Levi is closed-off and abrasive. His neutral expression often makes it difficult to ascertain what he's thinking.In the Japanese Visual Novel Burning Bright in the Forest of the Night that was released as an official bonus DVD feature, Levi's attitude towards Eren from their first meeting to the deaths of his squad is described in his own words.

Even before their first meeting, Levi and Erwin had researched Eren's background, so knew that the level of violence needed to avoid Eren's execution and get him into the custody of the survey corps was nothing the boy couldn't handle. " He is a headstrong youth. In his childhood, in his trainee days, he got into numerous fights and gained an unaccountable number of injuries.  He had even nearly gotten killed by traffickers.  He had even gotten swallowed by a titan...Yet, that violence and any other form of pain will never break the boy’s spirit. " - Levi in Burning Bright

Levi's past as a criminal living in the underground city and the parallels between his own upbringing and Eren's (both without mothers, both having to resort to violence to reach their goals) foster a sort of guardian/ward relationship between the two. In moments of uncertainty or doubt, Eren is therefore able to open up to Levi without being judged, because Levi recognises a similar rage within himself.

Their relationship can be confusing and does not fit neatly into the category of straightforward mentor/mentee because Levi does not hold back in his criticisms of Eren. Unlike Mikasa and Armin, Levi is unafraid to label Eren a 'monster' in their early relationship and consistently labels him 'brat' in casual conversation. Levi is an antagonist to Eren's doubts when required, but at the same time is also adept at soothing them. In his early days living with the survey corps Eren is ordered by Levi to get his titan shifting ability under control for the sake of humanity. However, when Eren is confronted by the rest of the squad and labelled a monster after he accidentally shifts, Levi places himself in front of Eren and takes his side, asking him if he is alright and later explaining to Eren why his squad reacted that way.

After their untimely deaths, Levi is the one to console Eren in his own way, stressing that there was nothing Eren could have done, and telling a wry joke in an attempt to lighten the mood. Although Eren's hero-worship of Levi lessens over time, he still respects him greatly and works far harder to impress him than any of his other comrades. Levi is known to be a 'clean-freak', and despite being in the midst of crisis Eren goes out of his way to help Jean clean up to meet the captain's standards, leading to his being labelled a "nagging housewife".

No one else at the table seems to care when Levi makes a comment about the job they've made of the cleaning, but Eren is visibly frustrated.

When Eren begins to suffer under the weight of all the titan-hardening training he's been subjected to, Levi is the one who notices and makes sure he gets a break, offering him a tissue and commenting on how "he probably used his titan powers too much". At the start of their relationship Levi had been apathetic and disinterested, but by the time it comes to the final battles Levi has shown enough concern for Eren at various points in the series that killing him outright like he had initially offered would be extremely difficult for him. If Levi was eventually tasked with killing Eren, Isayama's establishment of this dynamic leaves it impossible that he would feel nothing.

Fanon
Although Levi's age has never been officially confirmed, Isayama has stated that he is "suprisingly old" so it has often been suggested that he is in his early 30s. Because Eren is 16 (as of March 2016), this has led to controversy surrounding the ship, particularly in the western fandom. Despite this, the fanbase continues to grow, with some preferring to focus on the emotional bond between the two or choosing to reimagine the characters at different stages in their lives, and others stressing that the extraordinarily cruel world of Attack on Titan has given Eren an emotional maturity that almost matches Levis, so age is just a number.

The vulnerability of humanity in the face of titans and the brutal deaths of Levi and Eren's family and friends have led some fans to imagine that they would develop a romantic/sexual relationship overtime, finding comfort in one another. Both have violent tendencies and struggle with the weight of responsibility placed on them as 'humanities hope' and 'humanities strongest soldier', but there are also moments between Levi and Eren in canon that suggest they could overcome this in each others company. On a superficial level, Levi's cold demeanor and his treatment of Eren in episode 14 lend themselves easily to a sadomasochistic template. Eren's inner fire combined with the unwavering respect he feels for Levi indicates that there's not much he wouldn't do to satisfy him.

However, although the standard seme/uke dynamic prevalent in most yaoi animes suits Eren and Levi, fans like the pairing because both characters are well-rounded and neither are completely submissive or controlling. Eren could just as easily overpower Levi as he could him.

The popularity of the ship in Japan can also be attributed to the prevalence of Yūki Kaji (Eren's voice actor) and Hiroshi Kamiya (the voice of Levi) as Seiyuus. In addition to their voice acting credits in anime and gaming both have done a number of 'BLCDs' (boys love CDs), which have a huge market in Japan. A crossover of clips from these CDs has over 1 and a half million views on youtube, so although there is little to no probabibility of Levi and Eren ever interacting romantically in canon, the fan culture is encouraged by what are effectively fanon sex-tapes.

Levi and Eren's interaction in the courtroom scene is not the foundation of their relationship, and the popularity of the ship isn't exclusively sexual. A popular theme for Eren and Levi fanworks, and Attack on Titan fanworks in general, is that of reincarnation. (Examples alongside this are by Lena). After all of the suffering both have experienced at the hands of titans and humans alike, there is comfort in the thought that after death they could lead a happy life in the modern world.

Levi's status as captain in the canon world and the power dynamic this entails has also led to a number of alternate-universe fanfictions being written, the predominant theme being that set at a school (largely thanks to the official parody series 'Attack on Titan: Junior high').

Levi's cold and calm attitude in contrast to Eren's character has also led to a surge in works placing Levi as a company boss and Eren a distracted college student. Unlike the characters in 50 shades, Eren and Levi are accustomed to violence but are not abusive, they are familiar with their boundaries. Thus the artists who make connections between the two franchises such as in this '50 shades of Ereri' youtube trailerare clear in their belief that by incorporating Isayama's characters and the concept of reincarnation to the franchise they could "transform it into a meaningful one, with similar visual expression". As well as BLCDs and fan artists, Ereri shippers are also benefitted by the huge number of official works relating to the couple. Magazine spreads, advertisements for watches, and even Eren dreaming about Levi on an official CD track all contribute to the prevalence of the pair in the Attack on Titan fanbase.