RinLen

RinLen is the het ship between Rin Kagamine and Len Kagamine from the Vocaloid and Project SEKAI fandom.

Canon
The RinLen ship has been used in many different ways when it comes to songs. The two most popular ways they are portrayed is as siblings or as a couple. It is worth mentioning that there are songs that also portray them as rivals or just as friends. However, none of the songs that are produced for these two should be considered canon, because all Vocaloid are meant to be used as blank sleets.

RinLen Sibling/Family songs
The song "Childish War" by Giga-P is a great example of a song about sibling rivalry. The song is about Rin and Len making stupid and silly competitions. In the end they just continue arguing. Another good example of a song portraying them as siblings is "Gemini" by Dixie Flatline. This song’s is about them rekindling after an argument. Other songs include:
 * "Bring it On" by Giga-P
 * "Rimokon" by Wonderful☆Opportunity
 * "Daughter of Evil" by Mothy

RinLen Romantic songs
The song "Love-Hate" by Honeyworks is a great example of a song showing Rin and Len in a romantic way. The song is about Len trying to win over Rin but is failing miserably because Rin does not know how to accept his feelings. At the end of the song, Rin finally comes to terms with her feelings and accepts Len's confession. Other songs that include them in a romantic way are:


 * "Fire Flower" by Halyosy
 * "True Love Restraint" by Wakacha
 * "Straight Faced Science Girl" by Ningen Dokku
 * "Feathers Across the Seasons" by TeamOS
 * "Soundless Voice" by Hitoshizuku

Project SEKAI
In Project SEKAI, the Rin and Len's relationship differs in each unit's SEKAIs, although they are generally on good terms. Their relationship is most defined in the Street SEKAI, where they, despite often bickering, regard each other as partners and are important to each other.

Fanon
Although RinLen is considered one of the most famous Vocaloid ships, supported by many fans of the characters and which is associated with them in many ways, a noticeable number of people also view it rather negatively, given the original positioning of the characters as mirror twins. The enormous popularity of their incest shipping and complaints about it eventually even led to the fact that the authors declared fanon their relationship, leaving as their "official" concept only the statement about the mirrorness of the characters. Alongside this, it's already a common misconception that Rin and Len are canonical twins. The argumenting over the ship has gotten so bad in certain circles to the point where even the official Project SEKAI English Discord server banned the topic, due to "the server not containing the maturity needed to have conversations like this."

Because of this misconception, it often creates tensions between fans and typically leads to flamewars online. However, from that moment they are not twins anymore, as Crypton has stated that they are whatever the fans want them to be.

Twincest and forbidden love is still one of the most popular motives among RinLen shippers, even though some producers prefer to portray them as fateful lovers rather than twins. At the same time, this ship also gained immense popularity as a family one, many songs or fanon include the cute interaction of characters like brother and sister. This also seems to be the most popular reading of their relationship in the official media, although Project SEKAI has abandoned this trend by deliberately portraying their bonds as ambiguous.

One way or another, regardless of the portrayal of twins or lovers (or even both), RinLen is still one of the most popular in the franchise and for Rin and Len, due to the fact that they are the very first and famous "twin" Vocaloid project.

Trivia

 * Since the only canon describes Rin and Len as mirror images of the same person, some songs also describe them as genderbent versions of the same person. For example, one of RinKu's songs is about how Rin turns into Len in order to get a chance for a relationship with Hatsune Miku.
 * Considering how often the relationship between these two inspires producers to write dramatic and tragic stories, there is a joke in the fandom that any song with their participation will contain dark and evil elements (especially featuring Len dying).
 * There are also songs that betray Rin and Len as both a couple and as siblings. One of the most famous songs that betray them in this way, would be the parody song of Cendrillon, Adolescence by Signal P.
 * Their Kizuna Rank titles include: "Kagamine Combo", "Always Together" and "Heat Up".

Fandom
FAN FICTION