Personality

I put it in two categories the outside how other characters see him and the inside, his twisted mind and insecurity.

Seymour is very gentle and soft spoken person, when you first met him. You never hear the Maester yelling or screaming, always in a polite matter even when he shares his opinion on the world. He even proposed to Yuna, to give it a more of a soft touch to his personality. Untill the party confronts him with the death of his father Lord Jyscal and he becomes a unsend.

Seymour maybe very gentle on the outside but on the inside.
His twisted mind of how the world is so cruel. He feels a deep empathy for the sorrow that the people are holding in Spira, this why he want to save Spira's sorrow by becoming Sin. Like he said before you're fighting Seymour Flux "To destroy to heal Spira, I will be become Sin". Sin is the endless cycle of death and rebirth. The Summoner defeats Sin with the Final Aeon, the Calm will start and then Sin will be reborn again.

He can act like a Leader and a Maester but he is very insucre about how he looks. A good example of this is in the wedding scenes, Seymour gently touched Yuna and looked how beautiful she is. He even put a smile on his face before kissing her. When Yuna wiped her mouth in disgust you can see for a view seconds that he has being hurt by it but because he already is used to it, it doesn't matter anymore.