Miku is really like a sister to her! It was wonderful meeting a girl of her age from her culture here, and they can talk together about girl things, hang out in the baths together, and cook together - all that cute bonding and stuff.

Of course, given HOW they met, Nanoha also has a pretty protective streak over Miku, and wouldn't hesitate to defend her with her full power. She's like a little sister. Even if Nanoha's pretty sure she might be older than she is.