Hoster Blackwood, also known as Hos, is the third son of Lord Tytos Blackwood of Raventree Hall, and the younger brother of Lucas Blackwood. He is described as bookish, possibly why there is no mention of him fighting in the War of the Five Kings on the side of House Stark.
A Dance with DragonsEdit
Hoster is taken hostage by Ser Jaime Lannister to secure his father's good behavior after he bends the knee to the Iron Throne. Hoster takes the place of his sister Bethany, or his younger brother Edmund. Jaime doesn't think he takes the situation seriously enough. The deal is a compromise, as Lord Jonos Bracken wanted Bethany taken instead, as he knew that would hurt Lord Tytos the most.