Sleez was a former denizen of Apokolips.
Sleez was born in the lower levels of Apokolips. When he gained self-awareness and discovered his ability to influence people's baser desires, he fell into the employment of Darkseid. When Darkseid rose in power, he began to tire of Sleez' petty powers and had him teleported to Earth, where he would fester in the sewers of Metropolis, waiting for the right opportunity to rise once more.
SupermanWhen the New God Big Barda took a wrong turn into the red light district of Metropolis, her purse and mega-rod were stolen from her. She followed the thief into the sewers, where he was captured and killed by Sleez' pet, an octopus-like creature known as an ash-crawler. He discovered the mega-rod and realised that the wielder would soon follow, leading him to the opportunity he had been craving.
When Big Barda appeared, Sleez used her mega-rod against her, knocking her unconscious. She is then stripped naked, forced to wear heavy make-up and a revealing outfit and brainwashed. Its implied that Sleez may have even sexually abused her. Superman arrives, this being his first confrontation with Barda, and manages to break Sleez' hold over her. When Barda threatens to kill Sleez, Superman stops her. The two brawl and Sleez traps them, brainwashing both of them now.
Sleez then films Superman and Big Barda doing things that would violate the Comic Code and sell the tape. The person who bought it, however, was an agent of Darkseid. Darkseid then showed the tape to Barda's husband Mister Miracle, his only reason for helping him seemingly because he still considers Sleez an embarrassment. Sleez then introduces Superman and Barda to his human associate, an adult film-maker, as a way to make a name for themselves in the porn industry.
Mister Miracle arrives on the scene just as shooting had begun. Sleez was without Barda's mega-rod and couldn't brainwash a fully-conscious new god, so he fled. The ash-crawler provided a distraction for Mister Miracle, but the much stronger Big Barda managed to kill the beast. Superman followed Sleez into the sewers, but Sleez had an escape plan. He struck a match, which ignited the methane in the air and caused an explosion. When the smoke cleared, Sleez was gone.
Years later, Jimmy Olsen came across Sleez, hoping to find out information on Darkseid. Before Sleez could tell him, however, he was killed by Infinity Man.