Walvar Rivers is a bastard son of Lord Walder Frey. He is one of Walder Frey's older bastards, around the same age as Ryger, another of Lord Frey's bastards.
Walvar's mother was a wet nurse whom Lord Frey took a special liking too. A year after laying with her, Walvar was born, and Lord Frey didn't hesitate to acknowledge him as his son, giving him the last name Rivers.
He was later sent to serve at the Twins along with his bastard and trueborn siblings, getting to know Stevron and Ryger the most. Though he was mocked by the lesser houses and even his own father for his status as a bastard, he always strove to prove his worth.