So as of now we have DiaMaru and ChikaYoshi. I'm thinking of either KanaRiko and YouMari or MaRiko and KanaYou........
Or maybe an angsty RubyDia.
Aha.. some incest
I was thinking in some kind of bullie relationship.
But that works as well.
I honestly only see it happening as a one-sided crush Dia has on Ruby but never told her (maybe the reason she put some distance between the two) and keeps her feelings concealed.