Bill Weasley is the oldest son of the Weasley family. He's a curse breaker for Gringotts Bank. He lives in Egypt; Mr. and Mrs. Weasley pay him a visit over the Christmas holidays, which leaves Percy, Fred, George, Ron, and Ginny in the castle over winter break. We don't know much about him yet, except that he got twelve O.W.L.s (a standardized test called the Ordinary Wizarding Levels) and was Head Boy his seventh year at Hogwarts. So he must be a pretty talented guy.