A Fairy is interchangeable between the classes of Bard and Thief. Here's how I normally do it.
Roll a Faerie Ninja, and remember to have a few points of Kirijutsu (so you don't lose it). Around level 5-6, when you are fighting groups that give you 1500+ xp, after (or even during) a fight, you can switch your Faerie to Bard (or thief) for a level almost every fight.
In using this method, by the time my other characters were level 10, my faerie had 120+ hp, and 100 on at least 2 stats per catagory. And all of the best mage spells.