John Simeon is not a well-known comic book creator, but if you meant Joe Simon, then the answer is Captain America. Joe Simon co-created the character of Captain America along with artist Jack Kirby, and the character made his first appearance in Captain America Comics #1, published by Timely Comics (which would later become Marvel Comics) in 1941. Captain America has become an iconic superhero, and continues to be a beloved character in the Marvel Universe to this day.