A demonic ghost taking the form of a tall traveling salesman. He approached several innocent people in New York (Eduardo 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 the idea to ask him to grant one more wish before he left: that he not grant her wish. Confused by this paradoxal wish, Duophanes took on his original form, letting the Ghostbusters capture him.
Duophanes is a slick salesman, always approaching people with his slogan & getting them to tell him what their wishes are. The way he shows their wish as they would like it to be granted in his briefcase also shows that he always knows exactly what they want, & the way he warps their wishes is just 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. & 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.
His human disguise also acts as a shield, protecting him from the Ghostbuster's proton beams & ghost traps. As a result, he has only one weakness: someone making a wish that can be used against him or that creates a paradox.