Zig Zagged: Miki's fighting ability seems to differ from day to day. One day,she's as strong as most adult fighters,another she's able to take down giants,the next she's just a normal kid,and the day after that she's at adult strength again.
Averted: Miki is a completely normal kid.
Enforced: Since Miki is very popular among fans, the author decide to give her more screentime. However the story is focus on battle and there is very little room for character without combat skill, so he made her a Little Miss Badass as well.
Lampshaded: "How the heck can a little kid punch that hard?! I can't even do that!"
Invoked: Miki's father groomed her as a fantastic fighter, but also encouraged her interest in 'normal', cutesy things, both to give her a relatively normal childhood and to ensure others might underestimate her.
Exploited: When looking for teammates, Agata tries to find the smallest, cutest girl around, because he knows she'll be the toughest too.
Agatha looks for the biggest and roughest and ugliest guy around to be his Big Guy because he can't imagine that cute little girl could be any help despite impressive strength.
And the fact that she's so strong for her size makes the team forget that there are things stronger that her. They have her try to deal with Grik. No Holds Barred Beat Down ensuses.
Alternatively, Miki starts off as cute yet weak, but by becoming stronger she gets a rougher exterior.
Miki firmly believes that her cuteness gives her physical strength. It turns out her confidence in her appearance and her confidence in strength are directly proportional to one another.
She then gets back up, says "My turn" and returns the favor.
Daniel helps them get along by reminding Agata that she's still younger than them while assuring Lemina that she doesn't need babysitting and helping Cecil cope with her nerves. All the while encouraging Miki to do her best.
Once Miki recovers she adds Waif-Fu and some kind of projectile to her arsenal.
Played For Drama: Nobody was aware that she was stronger than she looked, not even Miki — this only came out when she and her friends were about to die and the frightened girl flew into a berserker rage, attacking savagely and desperately.