Mizuki is relative a weak and easily affected girl in the start of the story, but slowly she started becoming more and more marture as the story picks up.

Her ever determinatino that Sho is still alive and would come back to them after Sho and Agito was "killed" on the Creator's ship shows us her devotion towards Sho.

Either by chance or her deep affections, she summoned Sho back with the Gigantic and thus saving them from Zector.


My Comments
Every male hero needs a girlfriend right? :) Who is he going to protect if there wasn't? Ants? Properly not.
In the earlier series we do see her in bathrooms or nuked a lot of times. But that took a change ( for the better ) in the later series where her character re-draw was much more superior than the earlier series.