Who is Bert Weiss from Drowning Ruth and what is their importance?

Bert Weiss is a swaggering, nose-picking eight-year old boy in the Lakeridge School, Bert carries around a greasy bag full of marbles, including a prized blue, which Imogene Lindgren is determined to win from him. She uses Ruth Sapphira Neumann to lure him into a game. This begins their life-long friendship.