Sora and Mimi
Although initially Sora wasn't quite sure how to befriend Mimi (their interests were so polarized it was ridiculous) they became still became friends rather quickly. Mimi has one of those amazingly sweet and friendly personalities that Sora absolutely couldn't resist. It never ceases to amaze Sora how much Mimi expresses herself. As someone who spends a great deal of time regulating her emotions and responses, someone as purely honest as Mimi was almost strange but definitely not unwelcome. Although Sora doesn't really cut loose around Mimi the way she does with Taichi, she really enjoys being patient for Mimi.
Mimi can be the sweetest thing on the planet one moment and the brattiest the next, despite herself Sora found this trait absolutely endearing and strove to protect the younger girl (sometimes even from her own ego).
Mimi was also how Sora noticed that she might have a bit of a complicated sexuality. Let's face it, everyone falls for Mimi sometime or another.