Our third main character, Silver is a big grey rabbit with a slow minded personality. He accompanies Hazel and Fiver on their journey to find a new warren but, along the way, he has little doubts that Hazel and Fiver actually know where they are going.
For his voice, I would give him a strong but withdrawn sort of voice in order to represent his large physique in a better light, as well as to show he's way of thinking when he has something to say. Which would be slow in this case!