KaiRin

KaiRin is the het ship between Kaito and Kagamine Rin from the Vocaloid and Project SEKAI fandoms.

Canon
Kaito and Rin are both From the Music software Vocaloid. They don't have and official dinamic or relationship as they are Vocaloids. Their relationship depends of the song that they sing together. They have in common that both are from Crypton Future Media, so they usually appear together in official stuff. They also appear in Project SEKAI.

Songs
There aren't many Kaito and Rin duets, but there are some songs were they have some kind of relationship. In the Story of Evil, Rin (as Rilliane) and Kaito (as Kyle) were engaged but Kyle broke his engagement because he fell in love with a green haired girl called Michaela. After that Rilliane ordered to kill every single green-haired girl from her kingdom. The song Queen Thumbelina talks about how many boys (Including Kaito) falls in love with Rin. There is also a video about that song that explains how Kaito fell in love with Rin. Kaito never had experience with girls before and he refused to date Rin, but once Kaito and Rin has a date, Kaito became obsessed for her.

There are also some song parodies/alternative versions that contain this duo. There is a version of the song Kokoro were Rin is a scientific who created Kaito. In this version of the song, Rin literelly gives her heart to Kaito in order to keep him alive, but Kaito brings Rin back to life at the end. In Oni Yuukai, Rin has fantasies with Kaito and she, Miku and Luka end up kidnapping him.

Fanon
KaiRin isn't that popular because both already have more popular ships. Even son, KaiRin is a moderately popular ship. Like KaiLen and KaiMiku, the ship is controversial because Rin is 14 and Kaito, Despite not having official age, looks and sounds like and adult. KaiRin shippers either try to acclaim that Kaito is ageless or just don't care about their ages while their voices sound well together. In AO3 KaiRin has a good amount of works for the average Vocaid ship from that page, +28 works, being the 3rd most writted ship for Rin and the 5th most writted for Kaito.

Trivia

 * Their Kizuna Rank titles include: "Warm scarf", "I'll play a prank" and "Let's play anytime"