Zak-Kul is a brilliant scientist of the city of Kandor, who went rogue and committed a number of crimes. He had figured out a way to enlarge himself using the element Illium-349, but was unable to implement it without destroying Kandor as he enlarged.
When Lois Lane was visiting the Fortress of Solitude for the first time, she accidentally knocked over the bottle city, releasing Zak-Kul, who was then able to enlarge himself. Using an Electronic Plastic Surgery Machine, he made himself look like Superman. He then fooled Lois into shrinking the real Superman, and took over Superman's life as an admired hero