Duophanes (also known as the Salesman) is a demon taking the form of a tall stereotypical traveling salesman.
According to Egon Spengler's Spengler's Spirit Guide database, Duophanes is a demon originating from ancient Greece. He reappears briefly every 300 years in various guises granting wishes. His essence is overlaid with how he manifests, making his human form a shield from attacks.
In 1997, Duophanes arrived at Grand Central Station around 4:00 and took an employee as his first victim. He traveled to Central Park and a flea market on 63rd and approached several innocent people in New York (Eduardo Rivera included), offering to grant their wishes only to twist them into something horrible. He was finally stopped on his way out of town by the Ghostbusters when Eduardo gave Kylie Griffin the idea to ask him to grant one more wish before he left: that he not grant her wish. Confused by this paradoxical wish, Duophanes took on his true form, letting the Ghostbusters capture him. All transmogrification's were reversed and his victims were restored to normal.
Duophanes is a slick salesman, always approaching people with his slogan, and getting them to tell him what their wishes are. His method involves showing their wish as they would like it to be granted from within his briefcase, and at their whim, he then warps their wish according to his sadistic choice.
Once he hears someone's wish, Duophanes can grant it using a release of power that he keeps in his briefcase. In addition, the creatures created/transformed by his power are not ecto-based but actually real, making proton beams ineffective against them. Duophanes can twist the wish only based on the exact wording of it. These wishes were classified as Class 6 Spectral Transmogrifications.
If someone originally turns down his offer to make their wishes come true, he demonstrates the ability to tempt them against their senses until they make a wish, often with his slogan "No currency required, no payment do I need. Satisfaction completely guaranteed."
Worse still is that once he grants enough wishes in an area, Duophanes will try to leave it to find a new place to terrorize. If he manages to do so, then the effects of all the wishes he granted there become permanent. His human disguise also acts as a shield, protecting him from the Ghostbuster's Proton Streams and Ghost Traps. As a result, he has only one weakness: someone making a wish that can be used against him or one that creates a paradox.