In the books, he is described as having silvery grey fur and yellow eyes, but in the show, he has yellow fur.
A Game of Thrones
A Clash of Kings
Summer becomes even more bonded to Bran Stark, to the point that it seems Bran Stark can transport himself into Summer's body while he is asleep (making him a warg.) He learns that this is significant from Jojen Reed. He also regularly threatens the two boys of House Frey who have come to stay with the family, using Summer to make up for his injury.