Mostyn is the first king of Talys, who united the island tribes. He is the father of Caeda and is friends with Lorenz of Grust. He hid Marth in Talys after his flight from Altea, and was rescued by him when pirates under Gazzak attacked the castle.
He is not mentioned by name in any of the games, his name comes from supplementary materials.
Mostyn's Portrait in Shadow Dragon and the Blade of Light.