Nina is the exiled princess of Windia. She's in exile because her black wings are considered an omen of disaster back home. She'll join you after you help her save her sister and kill the demon that's obsessed with her.
Fighting/Defense -- Neither of them are worth a damn. She has what amounts to the worst attack of the whole party. She has low HP, and poor defense to go with it. She's fast, though. Her weapon is the ring, and her armor is limited to the various robes and other relatively light gear.
Special -- Will. 3 things can happen -- she regains 16 AP, she regains nothing (most often), or she regains 32 AP (if she's nearly out). Useful early on, but Drain is more reliable later on.
Magic -- She can learn every attack spell in the book (with the exceptions of Sap and 8.0), and most of the hindrance spells (except Hush and Idle). Her low attack doesn't matter much, since she'll either be using a spell or an item that produces a spell effect (most of the rings do) to attack. Typhoon and Tornado can be learned only by Nina. Only Bleu has as much AP as she does.
Shamans -- Wind or Wind + Fire or Water results in a color change and a 50% vigor boost. Devil results in a big AP boost. Wind and Holy results in the Angel Queen, and a 50% boost in AP -- if she had 300 AP before the combo, she now has 450 AP. Her special changes to Banish, which forces enemies who aren't bosses to flee. It can fail, but this is a very rare occurence.
Field -- Once you have access to the Great Bird, Nina can summon it while you're traveling the world. Just press A. In dungeons, Nina can fly out of a pit that she fell into. This prevents the party of falling in. There's only a few areas where this is of any use, though. The Angel Queen can summon the Great Bird and fly out of pits.
By level 40 -- The only spells she doesn't have are Bomb, Fireball, and Missile. Since she can cast BoltX, that doesn't matter. She doesn't fight well, but she can cause great destruction with her magic. Her shamanized form is great. Her HP is still low, so watch it. Finally, her Wisdom attribute will have reached the max of 255 by now.
Note: She won't learn the three spells mentioned above until her level is around the mid 50's.