Ben Jackson is THE most forgotten-companion of all time.
He was a Royal Navy seaman, and played by the handsome Michael Craze. Ben was a companion of the First and Second Doctors, and appeared in 36 episodes from 1966 to 1967.
Michael Craze died in 1998 from a heart attack. He was just 56 years old.
Only a single serial involving Ben Jackson survives in its entirety. All the others are incomplete. The BBC wiped its videotape archives in 1972 to "make space". Surviving episodes are usually found in Africa, Australia, New Zealand and the like... or were taped at home by fans (literally aiming a film camera at the TV screen).