Yuna is the secondary protagonist in Final Fantasy 10 as well as the main protagonist in Final Fantasy 10-2. She is the love interest to Tidus.
She is the daughter to Lord Braska, who defeated Sin 10 years ago, bringing the Calm. She began her pilgrimage starting in Besaid Temple where Tidus eventually arrives at. Because of him unaware of the way Spira works, due to him living in Dream Zanarkand, he rushes into the temple after hearing she had not returned for several days. When he reaches the Chamber of the Fayth, she comes out and collapses. Tidus falls in love with her in an instant.
He comes along with Yuna and her group and aided in fighting monsters. Yuna decides to make him one of her guardians. When they reach Guadosalam, Seymour Guado, the Maester of Guadosalam, proposes to Yuna, telling her their union would give Spira hope. She goes to the Farplane to think of it when Jyscal Guado, Seymour's father appears. Yuna sends him, but he leaves her a sphere.
When they reach the Thunder Plains Travel Agency she views the sphere and learns that Seymour murdered Jyscal and plans to destroy Spira. She hopes to marry him and hopefully get him to turn himself in while continuing her pilgrimage. They make it to Macalania Temple to prepare for the wedding and get a new Aeon. Tidus and the other guardians discover the sphere and head out to help Yuna. When they reach her, Seymour decides to kill her guardians, forcing her to fight too. When he summons Anima, Yuna is able to defeat it by summoning Shiva. The group then successfully kill Seymour, but before he can be sent, the Guado call Yuna a traitor and chase the group out of the Temple. They fall beneath the ice and land on Sin who teleports them to Bikanel.
The group find her in Home, home to the Al-Bhed, a race of peaceful humans that many considered heretics because they continue to use machina. Tidus learns that if Yuna gets the Final Aeon, she will die if it destroys Sin, devastating him. The Guado invade Home and capture Yuna and get her sent to Bevelle. Cid, Yuna's uncle and Rikku's father, has the group board an airship and destroy Home. They make it to Bevelle and are forced to witness Yuna marry Seymour, now an unsent. Yuna manages to escape to the Temple and everyone learns of Yevon's corruption, using machina themselves. The group break into the Chamber of the Fayth, no longer caring about Yevon's rules and save Yuna. They defeat a mutated Seymour and escape Bevelle, now being labled as traitors. Tidus and Yuna share a kiss in the Macalania pond.
Despite losing the respect of Spira, Yuna decides to continue her pilgrimage. They make it to Yunalesca, the first to defeat Sin and guardian of the Final Aeon. Yuna decides the spiral of death needs to end and they destroy Yunalesca, ending what many in Spira believed to be the only means of destroying Sin. They head back to Bevelle and upon learning what they did, Maester Mika sends himself (he was an unsent). They learn from Bahumut's Fayth that the true key to destroying Sin is to destroy Yu Yevon, and that Jecht (Tidus' father and current incarnation of Sin) can control Sin if he listens to the Hymn of the Fayth.
Using this knowledge, all of Spira sings the Hymn of the Fayth allowing Jecht to lower Sin's defenses. Cid's airship is able to take down Sin and the group goes inside. They have a final battle against Seymour and finally send him. They then encounter Jecht, who turns into the Final Aeon. After defeating the Final Aeon, Yu Yevon appears and takes control of each Aeon Yuna summons. By defeating each Aeon, the Fayth lend their aid to the group until Yu Yevon has no more Aeons to possess. Yu Yevon is easily defeated and Yuna becomes the High Priestess of the Eternal Calm. Because the Fayth no longer dream, Tidus is removed from existence and his spirit enters the Farplane, but not before Yuna tells him she loves him.
Final Fantasy X-2
In the sequel, Yuna is part of a group of Sphere hunters called the Gullwings after learning that Tidus could still be alive somewhere. She is a lot stronger in this game, starting off using the Gunner Dressphere to help fight. She eventually learns that the man she saw was not Tidus but is in fact the main villain of the game, Shuyin, who was a vengeful spirit from the real Zanarkand that the Fayth may have used to create Tidus. Shuyin wished to destroy all of Spira in retaliation for the death of his lover, Lenne, who looked almost like Yuna. Shuyin's planned to use Vegnagun for this purpose.
Yuna and her group destroy Vegnagun and fight against Shuyin. After defeating, him, Lenne appears out of Yuna's Songstress Dressphere and comforts Shuyin, easing his tortured soul as they fade away.
If you complete enough of the game, the Fayth reward Yuna by dreaming Tidus back allowing the two to reunite.
In the first game, since Yuna is a summoner, she has a lot of healing abilities and can summon powerful entities called Aeons.
Whenever Sin destroys a village, Yuna can perform the sending, allowing the dead to reach the Farplane so they can't become fiends. She can also send unsent, which are powerful undead beings that don't become fiends.
She mostly loses her abilities in the second game, but is able to use Spheregrids and Dresspheres like Rikku and Paine can. This basically means her abilities are based on what Dressphere she uses.