Nina is always the princess of Wyndia, which is always a kingdom in whichever world the Breath of Fire game takes place in. She is a powerful mage, employing many types of magic, but she mostly uses white magic. i.e.: healing. She has wings, and she can fly (with varying amounts of skill depending on the game) In Breath of Fire IV she has a sister named Elena. She is a "good guy" and uses her magic to aid her friend Ryu (the hero of each game). They are part of the group which has come to be known in BoF circles as the "Destined Ones." There is one Nina in each set, just like there is one Ryu. The Ryu and the Nina are always something of an item, although it is relatively sublime. Many generations pass between each Destined group.
