Floyd

Uncle Floyd (died 2009) was the uncle of Leonard and Michael Hofstadter, as well as their sister.

His older nephew, Leonard, recalled that Floyd was the only person who had been affectionate to him growing up, and was upset to learn from his mother Beverly that his uncle had died from what appeared to have been a heart attack. ("The Maternal Capacitance")

It need to be noted that it's not known if Floyd was actually a Hofstadter. If it was Leonards Mothers Brother then he is unlikely to be Hofstadter... :)