Seymour is that annoying Yevonite punk who Just Won't Go To The Damn Farplane Even Though You Kill Him Multiple Times. I want to know why the Sending is such a long dance, because if it were shorter, the world could have been rid of him a lot sooner.

Seymour is one of the Big Baddies of FFX. Except he's not actually that bad - he's just really annoying and wants to destroy the world by becoming Sin. He hopes to do this with the help of Yuna, and asks her to marry him. At first, no one knows what Seymour's really up to. Yuna is impressed by his aeon Anima and respects him as a maester of Yevon. Then, however, she stumbles upon Lord Jyscal's sphere and learns that Seymour killed his own father. She goes to meet with Seymour at Macalania Temple, hoping to convince him to turn himself in, as an exchange for marriage. Seymour doesn't agree, and in the end, the party defeats him.

He comes back as an unsent and kidnaps Yuna to force her into marrying him. Yuna tries to send him, but fails. Then, he comes after them two more times prior to their arrival in Zanarkand. He wants Yuna to use him for her Final Aeon so that he can become Sin. He wants to destroy the entire world, thus ending all of Spira's pain and "saving" the people. I suppose he has good intentions, but really bad methods. At any rate, he Must Be Stopped and at the very end, the party finally manages to get rid of him. Good riddance.

Seymour was someone who Yuna once respected. All in all, they don't really have a relationship per se so much as a cat-and-mouse dynamic; Seymour is trying to get Yuna and use her for his own selfish purposes.