Most Helpful Guy
I would suggest a surfer hippy kind of personally.
Sort of cool and laidback and real sweet to everyone.
But add a slight twist, whenever she sees someone she loves get hurt she turns into a murderous psychopath scary bitch.
And then after she gets her revenge or the threat is gone, she goes back to the sweetheart she was before.
I know this has been done before but that's the first thing that comes to mind.