THEIR STORY

The Beginning
When Tidus first arrives in Besaid, he learns that there is an apprentice summoner who has been in the temple for a day, praying to the Fayth of the temple. Worried that something may have happened to the apprentice summoner, Tidus goes into the Cloister of Trials against the wishes of the Yevon priests. Tidus makes it as far as the room just outside the Chamber of the Fayth, when the apprentice summoner, Yuna, emerges from the Chamber and announces that she has become a full-fledged summoner. Tidus looks on as the people of Besaid watch Yuna summon Valefor. That evening the villagers gather around a bonfire, where Yuna introduces herself to Tidus.
"I remember...That night, we talked for the first time. I didn't know it then, but after that night, everything changed. For everyone...For me..." -Tidus

She's Always Dancing
On the way to Kilika, the group stops in Kilika, where Yuna has to perform a sending as Tidus watches.
"I wished there would never be a next time. No more people being killed by Sin. No more sendings for Yuna. Everyone stood there watching her. It was strange, and somehow...horrifying. I never wanted to see it again." -Tidus
While in Kilika, Tidus has his first encounter with Sinspawn and begins to consider becoming a guardian.
Yuna: "I want to ask you to be my guardian."
Wakka: "Yuna! What? This is no time for jokes, ya? He may be a blitzball whiz kid, but up against fiends, he's a newbie."
Yuna: "Not a guardian, then... I just want him nearby."
Tidus: "What? What do you mean?"
Yuna: "It's just that, well..."
"A guardian is someone a summoner can rely on. Someone she can trust with her life. I wondered, did Yuna feel that way about me?" -Tidus

Just Whistle If You Need Me
In Luca, Tidus teaches Yuna how to whistle. At first, she can't make a sound, but she practices, and before they leave Luca, she's learned how to whistle. Whistling becomes a symbollic thing between the two of them that carries out through the rest of their story.
"Hey, use that if we get seperated. Then I'll come running, okay? Well, guess we should just stick together, then, till you can do it?" -Tidus
"If we should get seperated, just whistle. I'll come running, I promise." -Yuna

The Proposal
While they are in Guadosalam, Maester Seymour proposes to Yuna, claiming that it is their duty, as Maester and Summoner, to not only bring peace to Spira by defeating Sin, but to ease Spira's pain by bringing them joy as well.
"I had to stop myself from shouting: 'What's there to think about?' I wondered why none of the others ever asked Yuna, 'Do you love Seymour? Do you even like him?'" -Tidus

What Nobody Could Say
Yuna disappears after a Sin attack and Tidus and the guardians go to the Al Bhed's Home to search for her there. Upon reaching the Summoners' Sanctum, they find that Yuna isn't there after all, and Tidus learns more about what it means for a summoner to go on a pilgrimage and defeat Sin.
Tidus: "Why couldn't they trust guardians to protect summoners...The Al Bhed had no right stopping their pilgrimage!"
Rikku: "The pilgrimages have to stop! If they don't, and they get to Zanaraknd...They might defeat Sin. Yunie could...but then she... Yunie will die, you know? You know, don't you? Summoners journey to get the Final Aeon. Yuna told you, didn't she? With the Final Aeon, she can beat Sin...but then...If she calls it, then the Final Aeon's gonna kill her. Even if she defeats Sin, it will kill Yunie too, you know?"
Tidus: "Was I the only one who didn't know? Tell me why! Why were hiding it? Why didn't I know?"
Wakka: "We weren't hiding it..."
Lulu: "It was just...too hard to say."
Tidus: "Lulu! How could you? How could you? Isn't she like a sister to you? I thought you were family! Why don't you do something, Wakka?"

Macalania Woods
After rescuing Yuna from her wedding with Seymour and escaping the Via Purifico, Yuna and her guardians stay the night in Macalania Woods. Tidus follows her down to the lake and one of the most significant scenes between them ensues.
Yuna: "I always thought that this would be easier somehow. I thought that everyone would help me...with all my new friends together beside me. I've been trying so hard."
Tidus: "Maybe you're trying too hard.They told me...everything."
Yuna: "Everything? Well, so then... you know."
Tidus: "I'm sorry. It's just, you know... all those things I said. Like "Let's go get Sin!" Or about Zanarkand...I didn't know what would happen to you, Yuna. I guess... I hope it didn't make you sad. Forgive me."
Yuna: "I wasn't sad. I was happy."
Tidus: "Yuna. Just don't do it."
Yuna: "The pilgrimage?"
Tidus: "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. Wouldn't everyone be surprised?"
Tidus: "Yeah. Except Rikku. She'd be with you. Lulu and Wakka wouldn't hold out long."
Yuna: "Kimahri would say yes, too, I know. But Sir Auron..."
Tidus: "I'll make him understand, Yuna. It's the least I can do for you."
Yuna: "No, I should tell him. He deserves it. What'll I do if I give up my pilgrimage?"
Tidus: "Hey! Zanarkand! Let's go to Zanarkand! Not the one in Spira, the one I'm from. Yeah, we can all fly there. Everyone can go! Then we'll have a big party at my place!"
Yuna: "And then we could see Blitzball! Your Zanaraknd Abes would play! We could all watch you play, in the stadium all lit up at night! I'd cheer and cheer till I couldn't cheer anymore!
.....
Yuna: "I'd like to see it, someday."
Tidus: "Well you can, Yuna. We can both go! Yu..?"
Yuna: "I can't. I just can't! I can't go!"
Tidus: "Yuna..."
Tidus sees that Yuna is crying and he pulls her into an embrace. They kiss while Suteki da ne plays in the background. Afterwards, they are sitting on the shore, looking out on the lake.
Yuna: "Stay with me until the end. Please."
Tidus: "Not until the end...Always."
Yuna: "Always, then."

Yuna's Sphere
Yuna records a sphere saying her goodbyes to her guardians. She includes a special message to Tidus. We find out that she recorded this sphere soon after they left Luca, while they stayed at Rin's shop.
"I guess that leaves...The newest guardian. Star player of the Zanarkand Abes! You are...I am...Well, um...I'm glad...I'm glad that we met. We haven't even known each other long, but...It's funny. So this is what it feels like. It's a much more wonderful feeling than anything I had ever imagined. Wonderful...but it hurts, sometimes. I wonder. I..I just want to say, thank you for everything. Maybe...maybe that's why it hurts. When I... When I think about us never being together again at all...I'm afraid. No, I shouldn't say that. I'll do that part over. Um..."

Zanarkand
When Yuna and her guardians reach Zanarkand, they learn the truth behind the Final Summoning. To Tidus' relief, Yuna finally turns her back on Yevon's false beliefs and decides to find another way to defeat Sin.
Tidus: "I don't know, but I have to try. This is my story. It'll go the way I want it... or I'll end it here."
Yuna: "Wait. You say it's your story, but it's my story too, you know?"

The End?
After defeating Sin, Tidus begins to disappear. He turns to leave, and Yuna runs to him, falling right through him. She lays there dazed for a moment before standing up. In the US version of the game, she then says, 'I love you'. However, in the Japanese version, I've heard that she actually says 'Thank You'. The second one makes a little more sense when considering some of the plot of Final Fantasy X-2. Tidus comes up behind her and pulls her into an embrace for a moment before diving off the ship.
At the very end of the game, after Yuna gives her speech and the credits roll, we see Tidus underwater. He swims to the surface, and the scene cuts to 'The End'. Form your own opinions about the final scene.

Is It Really You?
In Final Fantasy X: Another Story, two years after the defeat of Sin, Rikku brings a sphere to Besaid to show Yuna. The sphere contains a scene of someone who looks like Tidus, trapped in a prison and talking about using a weapon to save the summoner. Yuna of course, has hope that this might be Tidus, and leaves with Rikku to join the Gullwings, in an attempt to learn more about the sphere and Tidus' whereabouts.

Reunion
One of the possible endings will reunite Tidus and Yuna. We find out that the scene at the end of Final Fantasy X is Tidus swimming to the surface of the ocean just off Besaid's shore. The airship flies overhead and the ramp opens as Yuna comes running down and jumping into the water to get to Tidus. They hug and exchange a few words before realizing that the entire island is watching them and cheering from the beach. The scene ends with them running to the shore to join everyone, and Yuna promising to tell Tidus everything that's happened since they said goodbye.

An Explanation
A bonus scene is added for those who achieve 100% completion in Final Fantasy X-2. Tidus and Yuna are in Zanarkand, and Yuna asks Tidus how he came back. Tidus told him that the Fayth brought him back. Naturally, Yuna expresses her concern that Tidus might disappear again, to which, Tidus reassures her that he will not, just as long as they take care of one another. When Yuna asks him if the Fayth told him that, he laughs and replies that he thought it up just then. After two years, they finally get their happy ending.