An ace Blitzball player, Jecht was a dream of the Fayth for Dream Zanarkand but eventually found his way to Spira. Realizing that his Zanakand was not a real place, Jecht allowed himself to become the Final Aeon to defeat Sin and eventually become the next one.
Tidus always hated his father for always mocking him and leaving, causing Tidus' mother to die of sadness. In truth, Jecht did love Tidus and was trying to toughen him up. Before becoming the Final Aeon, Jecht asked Auron to watch over Tidus. When Auron became an unsent, he traveled to Dream Zanarkand to fufill his wish.
Ten years later, Jecht became the new Sin but still maintained some will of his own and brought Tidus to Spira so Tidus could kill him. It wasn't until Auron reunites with Tidus at Luca does Tidus learn of Jecht being Sin.
When Tidus and the group enter Sin, Tidus finally reunites with his father, who becomes the Final Aeon and unwillingly attacks the party. Tidus and the group are able to defeat the Final Aeon and frees Jecht's spirit. Before Jecht disappears, Tidus tells Jecht he's happy to have him as a father. After Yu Yevon is destroyed and the Fayth stop dreaming, Tidus enters the Farplane and high fives Jecht.