Jecht, Tidus's father was a popular Blitzball player From Zanarkand.
After volunteering to be Lord Braska's Final Aeon, he gave up his life to help defeat Sin, thus bringing the Calm to Spira for ten years. As a consequence for his sacrifice, he became the new Sin. He is responsible for Tidus getting his first weapon, as Auron passed on his sword from father to son.
Jecht was the best blitzball player in Zanarkand, legendary for his Jecht Shot Mark III technique. While a great athlete, he was nonetheless a poor father to Tidus. He called Tidus weak, mocked him for crying and due to Jecht taking all of his wife's time, Tidus was mostly neglected by his mother. The real reason for him referring to Tidus the way he did was to toughen the vulnerable child up. But it backfired, as the boy grew up to hate his father for the way he treated him as a child.
One day when Sin visited the Dream Zanarkand, Jecht was swept to the real Zanarkand. Here he was imprisoned for being a mad drunkard, nobody believing he was really from Zanarkand, the real world Zanarkand having been destroyed a thousand years ago.
After he vanished, Tidus's mother died soon after, Tidus hinting to Yuna that it was down to a broken heart. He resented his father even more for taking his mother away from him for good.
When Summoner Braska, Yuna's father was about to start his pilgrimage, he recruited Jecht as a guardian, together with Braska's loyal friend, Auron.
Jecht and Auron were polar opposites. Jecht was all about having a good time and enjoying life, while Auron was strict about even the smallest matters (like recording a video for Braska's daughter, Yuna). Jecht becoming drunk during their journey didn't help matters. He struck down a shoopuf while being in a drunken haze. As a result Braska paid the shoopuf's master all his travelling money. Jecht swore that he would never drink anything but shoopuf milk for the rest of his life.
When Jecht realises he will never be able to return home, he decides to sacrifice his life to become the Fayth of the Final Aeon. Auron had begged both Jecht and Braska to turn away from Zanarkand and return home, after learning from Yunalesca that the Final Aeon wouldn't be a permanent end to Sin - meaning the sacrifice would be in vain. Jecht stays with his decision, making Auron promise him to go back to Dream Zanarkand and watch over Tidus as he grew up.
After defeating Sin, Yu-Yevon takes over the Final Aeon and Jecht's fayth with it. Ironically, Jecht became what he wanted to destroy, the new Sin. Jecht as Sin, visits the Dream Zanarkand ten years later, specifically to transport Tidus to Spira so his son may find a way to destroy Sin permanently.
Using the Hymn of the Fayth, a song known to have calmed down Jecht before, Sin is temporarily at peace. Yuna, Tidus and the guardians take apart Sin piece by piece, eventually making their way to the core, where Jecht is waiting. He confronts his son face to face, for the first time in ten years. After transforming into the Final Aeon and being defeated by the party, Jecht dies in the arms of his weeping son. After Yu-Yevon is eliminated and Sin is finally destroyed for good, Jecht is seen welcoming his son to the Farplane, with a high-five, symbolising them making peace with each other at last.