Tidus and Yuna:

After almost getting married to Maestor Seymour (which Tidus did not like), Yuna and her guardians - Tidus, Rikku, Lulu, Wakka, Kimarhri, and Auron escape the city of Bevelle.
Yuna is shaken up, and has lost her faith in Yevon. She goes out into the lake, leaving behind the others at the campsite, to do some thinking.
After awhile, Tidus goes out to check on her. He gets to the edge of the lake, seeing Yuna there standing waist deep in the lake. Tidus had just found out from the rest of the party members, that when they go to confront Sin, Yuna will die. Tidus, not being from this world of Spira, does not know anything. So when he joined the party to become one of her guardians, he encouraged her to fight Sin, thus ridding the suffering for all the people in Spira. In order for Yuna to defeat Sin as a summoner, she must call forth the final Aeon. In summoning the final Aeon to defeat Sin, she must sacrife her life. Upon learning the truth of what Yuna must do, Tidu tries to apoligize to Yuna and to comfort her.
Yuna hearing Tidus approach speaks out -

Yuna: I'd always thought this would be easier somehow. I thought everyone would help me...... with all my friends together beside me. I've been trying so hard.
(Tidus slowly walks into the lake towards Yuna)
Tidus: Maybe you're trying too hard. They told me....
everything.
Yuna: Everything? (Tidus nods his head) Well so then.....
you know.
Tidus: Yeah. I'm sorry..... it's just ....you know all those things I've said....Like "Let's go get Sin" or about Zanarkand..... I didn't know what would happen to you Yuna.
Forgive me.
Yuna: I wasn't sad. I was happy.
(Tidus takes a deep breath, dives under the water and swims a little ahead of Yuna, then floats on his back)
Tidus: Yuna.... just don't do it.
Yuna: The pilgrammage?
Tidus: Yeah that's right. Forget all about Sin, about being a summoner. Forget all that. You know live a normal life. Come on now Yuna, what do you say?
Yuna: Maybe I will.
(Tidus swims back to Yuna, then stands in front of her)
Wouldn't everyone be surprised?
Tidus: Yeah....except Rikku. She'd be with you. Lulu and Wakka wouldn't hold out long.
Yuna: Kimarhri would say yes too, I know, but Sir Auron....
Tidus: I'll make him understand Yuna. It's the least I could do for you.
Yuna: No, I should tell him. He deserves it.
(Yuna then floats on her back)
What'll I do if I give up my pilgrammage?
Tidus (excited): Hey! Zanarkand! Let's go to Zanarkand.
(Yuna looks puzzled)
Not the one from Spira, the one I'm from.
(Yuna nods, uh huh)
Yeah we can all fly there. Everyone can go. Then we'll have a big party at my place.
Yuna (also excited too now): Then we can see blitzball.
Tidus: That's right!
Yuna: Your Zanarkand Abes will play.
Tidus: Yeah.
Yuna: We could all watch you play in the stadium all light up at night. I'll cheer and cheer 'til I couldn't cheer no more.
Tidus: Right on!
Yuna: What about after the game?
Tidus: We'd go out and have fun.
Yuna: In the middle of the night?
Tidus: No problem! Zanarkand never sleeps. Let's go to the sea before the sunrise. The city lights go out one by one, the stars fade.... Then the horizons glows almost like its on fire. It's kinda rose colored right? First in the sea then it spreads to the sky then to the whole city. It gets brighter and brighter, 'til everything glows. It's really.... pretty. I know you'll like it.
Yuna: I'd like to see it someday.
Tidus: Well you can Yuna, we'll both go. Yu....
(Tidus turns around seeing tears roll down Yuna's face and dripping into the lake)
Yuna (breaking up in tears): I can't.... I just can't...
I can't go.
(Yuna can't hold back all the tears and starts crying in front of Tidus)
(Tidus seeing her cry, walks up to her, placing his hands on her shoulders)
Tidus: Yuna
(Tidus then takes her into his arms. They both look each other in the eye. Tidus then kisses her. They kiss for ahile while holding hands and swimming throughout the lake)

After awhile, Tidus and Yuna are sitting on the edge of the lake.
Yuna: I'll continue. I must. If I give up now..... I can do anything I wanted to and yet...... even if I was with you. I could never forget.
Tidus: I'll go with you. I'm your guardian unless.....
I'm fired.
Yuna: Stay with me until the end. Please.
Tidus: Not until the end.....always.
Yuna: Always then.

Does Yuna go off and fight Sin? Will she die? Does Tidus find his way back home, back to his Zanarkand world? Will Tidus and Yuna live happily together?
All answers will be answered when you finish the game. Until then I'm not spoiling the ending....well maybe when everyone's had the chance to see the ending.