KaiMiku

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

Canon
The KaiMiku ship has been used in a variety of ways, including mutiple romantic songs. There have even been songs invloving them in a love triangle with multiple other characters. A majority of these romance songs have similar themes to fairy tale stories such as Cinderella.

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. However, there are people who prefer to see the KaiMiku ship as purely platonic, due to both characters having more popular ships. Ships like MikuRin and LeKai are considernly more popular in the fanfiction department.