This is an ongoing process, as it's difficult to sit down and write every detail about what we know about Yuna's life. So, it probably won't be finished for a very long time, but as I add more I'll update and make note of it. In any case, enjoy reading what I have here now!
We first see Yuna when she emerges from the Chamber of the Fayth in Besaid, when she made her final transition from an apprentice summoner to a fully-fledged Summoner and ready to start her pilgrimage to Zanarkand. This is where her life truly began, on those steps inside of Besaid Temple.
A mysterious man had shown up in the sea sometime before that, and he had gone through the Cloister of Trials without authority to save Yuna, because she had been in there so long. And that night during the celebration of Yuna's summonerhood, she spoke to him for the first time. His name, is Tidus. And from then on, Yuna's life would change...
The next morning, Yuna and her guardians Wakka, Lulu, and Kimahri--along with Tidus, of course, were to begin the pilgrimage, starting with the temple at Kilika Port. Yuna was very eager to begin her journey, even knowing what it would lead to once they reached Zanarkand. She had a smile on her face, and a trunk full of gifts for the temples... which she left behind in Besaid, anyway.
They board the ship to Kilika, and Yuna still bore a smile. The people of Besaid said their tearful goodbyes, believing that this would be the last time that they would ever see Yuna again. And on the ship, Yuna was bombarded by people that wanted to meet the Daughter of High Summoner Braska (or Lord Braska, if you prefer.) Yuna seemed to have wanted to be alone, but Yuna is not the type of person to just walk away, as we all know.
But once she was left alone, she was greeted by Tidus and they talked some more. It was obvious then that Yuna took a liking to Tidus, as she clearly didn't have much to say, what with her comment 'The wind... it's nice.' and all. Their laughter broke the ice, though, and so began their first big conversation. Yuna mentioned that she'd been told that Tidus was from Zanarkand, that she believed him because of the things her father's guardian, Jecht, had told her. Tidus then told her of how his father's name was Jecht, and this news seemed to have made Yuna even more thrilled to have met him, as if it were the 'blessing of Yevon.'
Their conversation continued, up until Sin attacked. The ship tipped and she was slipping, that was when their hands met for the first time as he tried to keep her from falling. Their hands broke away, however, and Yuna was caught by Kimahri. This was their first encounter with Sin, which resulted in Tidus falling overboard and Yuna becoming even more determined to defeat Sin. "I will defeat Sin, I must defeat Sin." Their attempt at keeping Sin from heading to Kilika was futile, as all they were able to do was scare Sin towards Kilika. It destroyed the town and killed the townspeople, leaving families desolate and distraught, and in need of a Sending to take place to send their loved ones' souls to the Farplane, where they may rest in eternal peace.
The Sending was both a beautiful and a tragic thing to witness. Beautiful, because of Yuna's gracefullness, the sunset, and the Pyreflies. Tragic, because of the pain Yuna felt doing it and the reason it had to be done. By Yuna's heart and staff, the lost lives of the people of Kilika were freed of their pain and would not turn into fiends that would forever roam and feed off of the living.
The next morning, they were to head to the temple of Kilika to obtain the next Aeon in Yuna's training. In the woods, though, Yuna made an announcement--that she wanted Tidus to be her guardian. No one felt comfortable about the idea, and it concluded with Yuna's apology and their leave towards the temple.
On the way to the temple, they enjoyed a nice race. Unfortunately, it was put to an abrupt end by their meeting with a Sinspawn. They fought and defeated it, and headed to the temple. There, they encountered another summoner by the name of Dona. She made fun of Yuna because of the many guardians Yuna had, and also because of who her father is. Yuna stood her ground and told Dona that she felt honored having many guardians, and for Dona to leave them in peace.
Dona and her guardian/lover Barthello left as Yuna had asked, leaving Yuna and her guardians to the task at hand. Tidus thought about what Yuna had said, and Yuna stood smiling at him for a moment, and then went off to the Cloister of Trials with her guardians. Tidus wasn't a guardian yet, so he had to stay behind.
He went to go back to wait in the main area of the temple, only to be greeted by Dona and Barthello who had not left, but stayed to push Tidus into the Cloister of Trials. He had no choice but to go through, regardless, once again, of the rules. He entered the waiting area of the guardians and was met with an angry Lulu, a shocked Wakka, and as usual a silent but fierce Kimahri. They waited for Yuna to emerge from the Chamber once again, and as she did she fell into Kimahri's arms full of exhaustion once again. Dona and Barthello were there, somehow, and Yuna was taunted by Dona's cruelty.
Disregarding the remarks made by Dona, they made their leave. Yuna obtained her second Aeon and had taken another step forward in her pilgrimage. She made another tearful goodbye as they board the ship to Luca for a blitzball tournament for the Aurochs--Wakka's blitzball team, which Tidus was now a part of.
On the ship, Yuna talked with a blitzball team, the Luca Goers--a team that had taunted Wakka and Tidus when they reached the temple. Tidus interrupted the conversation, which seemed to call for the Goers to taunt Tidus some more, saying that he was an idiot. Yuna defended him, and talked of how he was the Star Player of the Zanarkand Abes. Her protests seem to do more harm than good, and they walked away laughing. Tidus and Yuna talked after that, about how Yuna truly believes him and that she would love to go to Zanarkand. The conversation ended, and Tidus got some thoughts... "Yuna, wanna go to my place? ...Right, as if I had a place to go to!" Clearly, some sort of connection was forming. But Tidus being a guy and all, you can't expect him to love Yuna that quickly!
Later, Tidus is haunted by memories of the past, and out of his anger he attempts to perform the Jecht shot. (At this point, it's up to you whether or not he gets it. I get it, so I'm going to mention the moment that he DOES get it.) Yuna had been watching and was impressed. The Aurochs were quite impressed, too. Yuna and Tidus talk then, about the Jecht Shot, Jecht, and Tidus' hatred for him. (There was probably other stuff in there, too, but I can't remember that far back, even though I just replayed the game...)
They arrived at Luca, and each team was announced. The Aurochs, having the reputation that they have, were announced as if they were losers. Tidus wouldn't take this for a second, because he was a winner. He took a mega-phone and defended himself and his team, taunting the Goers and saying that they won't be number one for long, "HA-HAaAa!" He laughed very strangely at the end. Everyone raised an eyebrow at him, most probably, but Yuna laughed.
After that and a lot of random blitzball things, Yuna had heard that Auron had been sighted (I think so, anyway, I can't remember) and they went to look for him together. Yuna couldn't keep up with Tidus very well, but she did her best. Along the way, she had one simple thought. "I hope we don't get separated..." She then heard a whistle, which startled her. She looked up to see Tidus, who ran over to her.
"What was that?!" Yuna probably hadn't heard a whistle like that before, and Tidus laughed at her and proceeded to try and teach her. He talked about how they would do that if they would get separated, but Yuna couldn't do it right away so she just stuck close to him.
They continued on and ran into two nemesises of Kimahri's (Kimahri is always with Yuna, after all. He's even there later when--never mind.), and amidst all the excitement, Yuna was kidnapped. The blitzball game had started, and it appeared that the Al Bhed Psyches had kidnapped her and wanted the Aurochs to throw the game in exchange for Yuna's return.
Not as if they'd go that easy, so Tidus, Lulu, and Kimahri went to her rescue as Wakka held up as best he could. After much fighting, Yuna was saved--only right after Tidus had found out about her Al Bhed lineage, which was to be kept secret.
Fiends attacked Luca, and thus was Auron's entrance and Seymour's showing off of his Mommy's Aeon. Yeah. After that, there was... something about finding Auron... Tidus and Auron talk about how Sin is Jecht and, uh... Auron becomes Yuna's guardian, yada yada... Yeah.
Yuna had spent her time practicing whistling, and showed Tidus. Due to the conversation he'd had with Auron, he wasn't feeling so happy, and Yuna begun to teach him how to smile through the pain. It turned into a lunatic-type episode as the two 'laughed' very loudly, then they burst out into real laughter at how silly it was. The conversation ended with a single quote from Yuna, "If we should get separated, just come running, I promise."
Then they turned to the guardians, who were, "afraid you guys might've gone crazy," as Wakka put it. With that, they went on to the next destination; Mi'ihen Highroad.
Will finish later! Bear with me, please!