Kull-Ex was son of a Kryptonian inventor who was about to design a new kind of vehicle on Krypton, however the blueprint was lost and shortly thereafter, Superman's father Jor-El created a similar device by pure coincidence. The vehicles (dubbed 'The Jor-El') became popular among the people of Krypton. Kull-Ex's father was furious and believed that Jor-El had stolen the design for his personal gain. On his deathbed, he made his son promise to get revenge on Jor-El.
Years later, now living in the Bottle City of Kandor in Superman's Fortress of Solitude, Kull-Ex renewed his plans to get revenge against Jor-El. However, since Jor-El was long dead, he targeted his son Kal-El (aka Superman) instead. He built a device to swap places with Superman, and created a lifelike mask to pose as the Man of Steel. He then discredited the hero on Earth, ruining his reputation with poorly-executed super-feats, and wanton destruction. When Supergirl learned the truth, she freed Superman from Kandor. While the miniature Superman kept Kull-Ex busy, Supergirl and Krypto lifted the Fortress of Solitude into space so that they could get light reflections from Krypton to reproduce the events that happened the day that his Kull-Ex's father's plans went missing. Understanding the truth, Kull-Ex ended his vendetta against Superman, revealed his actions to the people of Earth and allowed himself to return back to Kandor and restore Superman to his regular stature.
