Olyvar Frey

Olyvar Frey is an unseen character in Game of Thrones. He is one of the many sons of Walder Frey and is a squire to King Robb Stark.

Season 1
When Catelyn Stark negotiates with Walder Frey to gain permission for her son Robb Stark and his army to pass the The Twins, Walder demands (amongst others) that his son Olyvar will become squire to Robb, and that he will later be knighted. Robb accept these terms.

In the books
In the Song of Ice and Fire novels Olyvar's squiring for Robb is also part of the bargain struck with Walder Frey.