The first superheroes Lee and artist Jack Kirby created together were the Fantastic Four. They became popular which led Lee and Marvel's illustrators to create new heroes. Again working with Kirby, Lee co-created the Hulk, Thor, Iron Man, and the X-Men; with Bill Everett, Daredevil; and with Steve Ditko, Doctor Strange and Marvel's most successful character, Spider-Man. As a result, Lee created a shared universe between the fictional heroes. Lee and Kirby combined several of their newly created characters together into the team title "The Avengers".