[Genten]

Unfortunately, we don't know too much about Miki yet (and we can't see into her thoughts like we can Madoka) so it's unclear what role she'll play in the future of this series.

Spoiler for Long Miki Interpretation:

Right now we know that Miki cares deeply about her friends and is very energetic. She's more than willing to help Mami in her missions: she is the one that brought a bat to the excursion that Mami brought them along on. She also seems more interested in what Mami has to say: while Madoka is still interested she doesn't show the outward enthusiasm that Miki does. While I wouldn't say that Miki wants to be a magical girl more than Madoka there is a certain energy she has that Madoka lacks (probably due to Madoka's timid personality, though).

What implications does this show about Miki? Well, it's really hard to say. At one point we see Miki pondering why the two of them were chosen to become magical girls. Whether this is merely a moment where she ponders their future or regrets being chosen by Kyubey is unclear. This is probably what underlies Miki's personality: an insecure person who is unsure of their future. Maybe this is why she wants to become a magical girl: she has no future in her current position and desperately wants to become a magical girl.

This would explain her attachment to Mami and her untrust towards Homura, even though it's really just on Mami's word. To Miki, Mami is living the dream: a magical girl helping people, someone with a sure future. This attachment could cause a rift between her and Madoka later on. Obviously, this is all speculation. But what role Miki has to play in this series is a mystery.