Seymour is eventually absorbed by Sin, after being defeated by Yuna and her guardians for the third time. He justified it by saying that he was releasing Kinoc of the burden of life. He also murdered Maester Wen Kinoc, when he felt that Kinoc was becoming too nosy. He also kills most of the Ronso when they try to stop him from reaching Yuna on Mt. He did so out of revenge, for Jyscal betrayed Seymour by banishing him and Seymour's mother to Baaj temple. Seymour is also responsible for the death of his father, Maester Jyscal Guado. Yuna does marry him, but only to leave him at the altar after trying to send him to save her friends and obtain the aeon Bahamut. His devious intentions become clear - he wants to marry her so that their bond as husband and wife will mean him becoming the Final Aeon, thus going with his wish to become the next Sin so he can finally destroy Spira. It appears he is doing it so the people of Spira have something to celebrate, as he knows he will not have a long life with Yuna because she has a sacrifice to make. He asks Yuna to marry him, but it is unclear what his intentions may be. He later shows his twisted thoughts, wishing to destroy all of Spira and save everyone from the suffering that Sin brings. Against the teachings of Yevon, he supports the Crusaders in their Machina battle against Sin. He summons the aeon Anima at the blitzball match to protect everyone from the fiends, but this is a display of power as he was the one who let the fiends into the stadium originally.
However, the player slowly gains insight into the twisted thoughts of Seymour through his actions throughout the game. He started his plan by murdering his father, Jyscal and taking his place as leader of the Guado and a Maester of Yevon so he could rise to power. Seven years later, Seymour went to Zanarkand to have Anima's statue transported to Baaj Temple so he could revise his plan to become Sin itself and destroy Spira. At the age of eighteen, Seymour's exile was over and he was welcomed back into Guadosalam where he became a priest. Due to his lonely and isolated life, he began to have twisted views on life and death. Seymour was deeply at loss due to his mother's death and returned to the Baaj Temple where he lived alone for three years. Seymour's mother gave her life to Yunalesca so that she could become a Fayth to Anima, believing that it was the only way that she and Seymour could be accepted by Spira. Because of his heritage, Seymour was seen as abomination by humans and Guado so Jyscal decided to have his wife and son exiled to the Baaj temple. A major antagonist and temporary playable character in Final Fantasy X, Seymour is half human and half Guado, and the son of former Maester Jyscal Guado and a human woman in a an attempt to strengthen the relationship between humans and Guado.