Crypton Future Media do note assign relationships between their Vocaloids. They prefer the producers to be able to make songs with as little influence from the makers as possible to allow the producers ultimate freedom to spawn creativity.
Also, it is worth noting that Kaito is a Vocaloid and Hatsune Miku is Vocaloid2. Kaito was also set up originally to be Meiko's counterpartner. Miku has no counterpartner at all herself as Crypton did not repeat their trends for vocaloid2 that they followed in Vocaloid.