Mopee was a magical imp who gave The Flash his Super Speed powers during the Silver Age. Mopee supposedly used something that Barry Allen owned to provide him his powers as The Flash. Though over the years it is been widely believed that this story of The Flash's origins of his powers has not been considered as part of the continuity of The Flash storyline since it conflicts with the original origin of The Flash where Barry Allen receives his powers in a freak accident. This has left much of what Mopee is and was as a character a Mystery.
The Life Story of the Flash references this character by saying that Barry Allen was inspecting a drug nicknamed "Mopee" on the night he received his powers.