Florapaw appears to be mostly right about this.
This was Silver, a nephew of the Threarah, who was serving his first month in the owsla.
Hazel could not help feeling relieved that Bigwig had brought only Silver - a quiet, straightforward fellow who had not yet really found his feet among the veterans.
[...]
"You know Silver, don't you? asked Bigwig, breaking in on his thoughts. "Apparently some of the younger fellows in the Owsla have been giving him a thin time - teasing him about his fur, you know, and saying he only got his place because of the Threarah. [...]
Even then, it's only "some of the younger fellows in the Owsla". Holly, upon arriving at Watership Down, says he is glad to see Silver there.
Silver in the book proves his worth on several occasions. Just before the rabbits go into the bean-field he is (apart from Hazel) the only rabbit awake in case of danger. Bigwig asks him to fight Strawberry when he is too weak due to having been caught in the snare (although fortunately no fight happens, and Strawberry joins them). Silver also helped in the rat fight and, like Flora said, he is the only rabbit of Holly's group to make it out of Efrafa all right

In the film, however, he seems...different. He complains quite a bit, and as far as I can remember, all mentions of him being in the Sandleford owsla and being the Threarah's nephew are omitted.