Jeff Mantell appears in The Santa in the Slush.
He was a pickpocket that Kris Kringle found on the street. He was given a job as a Santa Claus impostor. When he continued stealing money from bystanders, Kris confronted him and Jeff killed him.
Later, Booth and Brennan found blood on Jeff's bell, but he convinced them Santa impostors often swapped bells. Finally they had to check each Santa's rear for Chinese food on the trousers. Jeff was found guilty and he was beaten to the floor by his fellow Santas for killing Kris. Booth then arrested him.