Ryger Rivers

Ser Walder Rivers is a minor character in Game of Thrones. He is played by guest star Bryan McCaugherty and only appears in the ninth episode of the first season. He is a bastard son of Lord Walder Frey, the head of House Frey and Lord of the Crossing.

Season 1
Ser Walder is present when Catelyn Stark visits Lord Walder Frey to negotiate passage for her son's army across the trident. Like his trueborn half-brother, Stevron Frey, Walder admonishes his father to remember his courtesies. Lord Frey dismisses his son and reminds him that he's a bastard born of a milkmaid.

In the books
In the Song of Ice and Fire novels, Ser Walder Rivers is the eldest and most prominent of Lord Walder Frey's bastard offspring. The lines spoken in the episode, however, are spoken in A Game of Thrones by another of Lord Frey's baseborn children, Ryger Rivers, who is described as being young.