Marcel Costas is a gay businessman and playboy and an accomplice of serial killer and con artist Bernard Fremont. He'd assist Fremont and his gang of robbers and killers in drugging wealthy clients Fremont planned parties for, then strip their homes of valuable goods. They'd also clean up Fremont's messes, like killing biographer Wesley Banyon, who was about to portray Fremont in a more grotesque light.
When accomplice Hilary Marsden struggled to properly drug Russ and Monica Corbett whine robbing them, Major Case was assigned. When Banyon's apartment was found stripped, the detectives found Banyon's head in the fridge, which Costas failed to dispose of properly. As a result, another accomplice Mala Marsden, strangled Costas to death after Fremont drugged him to get rid of him as a liability. (CI: "Slither")