KaiMiku

KaiMiku is the het ship between Hatsune Miku and Kaito from the Vocaloid fandom.

Canon
Miku and KAITO are characters from the vocal synthesizer application VOCALOID, and this do not have any official relationship, personalities, or backgrounds; so, most of their characterization is left to songs and semi-official materials such as video games.

Songs
The KaiMiku ship has been used in a variety of ways, including mutiple romantic songs, like Cantarella, where Kaito has a obssesive crush with Miku and envenom her in orden to make her his, and Cendrillon, where Miku is forced by a fairy godmother to kill Kaito at 12:00, but end up falling in love with him after dancing together, and after she stabbed Kaito at the end, she can still feel him with her, and muses on how they want the moment to last forever. There have even been songs invloving them in a love triangle with multiple other characters, like ACUTE, where Kaito has a love triangle with Miku and Luka, and Miku, feeling pulled, tries to kill Luka, but Kaito sacrificace for her.

Project Diva
In the Project Diva series, they have plenty of interactions. Typically, they are shown as friends. However, Project Diva has included plenty of songs involving them in romantic songs. As an example, Cendrillion by Signal Pis by far the most well known romantic song between the two in the Project Diva series. Even songs that don't feature the two of them in a duet together, Project Diva has songs that will recommend matching modules of the two of them (Paris Movie Girl by apfel note as an example).

Fandom
FAN FICTION

Fanon
KaiMiku is an extremly popular ship within the Vocaloid fanbase. Fans fell in love with how well their voices mixed together. Along with their voices sounding well blended together, their designs also match due to them both having blue hair. Since their first duet, fanfiction, doujin, and plenty of fanart has been created for the two.

Despite Kaito not having official age, he is mostly seen as an adult because of his voice and design, and in the unoffical manga Hatsune Mix, Kei protays him and Meiko as middle aged people. Since Miku is 16, the ship has become very controversial in the Vocaloid fandom, and the huge amount of rival ships like Rinku, Lenku, Negitoro, KaiMei, KaiLen, and GakuKai (which are all more popular in AO3) are another reason of why this ship isn't as popular as before. Anyways, many people thing that Kaito's age is still up to you since Hatsune Mix isn't official and you can ship them with wathever you want, other people just don't care about the ages and only care about how nice they look and sound togethr, specially since there is some popular ships like Negitoro and KaiLen with some age gap too.

It is the 4th most popular ship for both Kaito and Miku in AO3 and it is the 8th most popular Vocaloid ship in general.

Trivia

 * The first thing that made Kaito popular is this couple of videos abut him and Miku in NicoZon: link1, link2