Yuna is half Al-Bhed, a race of people in the Final Fantasy Universe. She is from Spira and also a summoner. Her father Lord Braska was also a summoner. Her love interest in the game is Tidus, who is from Dream Zanarkand.
In FFX she is the weakest in the party but her Aeon makes up for it. It's best to save her for a last resort as her healing abilities are helpful. However if you are in a boss fight it's a good idea to use her as her Aeons do the most damage.