[polka-dot-jewel]

I think Mika explained their relationship in a blog last month...something along the lines of this:

Ayu was super stressed out one day and ended up leaving her staff and all of them so she could have some alone time. She found Mika (or vise-versa; I'm not sure if they met previous to then), and Ayu poured her heart out to her. Mika said something along the lines of "it's okay, it'll be alright" and Ayu felt better and went back to her staff. And they've been great buddies since.

From what I've heard, the "Dearest" lyrics are inspired by that one event...