Van Cleef was leader of a Rocket Men group.
On Steven Taylor's 21st birthday, he attacked his spaceship. He had a fight with the Steven from the future (although he did not know it at the time) which he thought was one of his men (Steven had stolen this uniform from one of his men). He thought he had shot the disguised Steven fatally but Steven had a book in his pocket. Steven then sabotaged Van Cleef's jet pack, sending him flying into space to his death, thus saving Steven's past self. (AUDIO: Return of the Rocket Men)