Dr. Janosz Poha is the (former) secondary antagonist of Ghostbusters II. This ??? (European) art restoration expert who is in charge of the Manhattan Museum of Art in New York City was the right-hand man of the 16th century Carpathian tyrant Vigo after agreeing to help the evil spirit in his quest to be reborn, deciding that Dana's baby would make a perfect host as he was madly in love with her and quite liked the idea of being the adopted father of a "god".
He was portrayed by Peter MacNicol, who later went on to play Fenwick in Recess: School's Out!.
However Janosz was not prepared for the intervention of the Ghostbusters and in order to aid his master became possessed by Vigo, gaining supernatural powers but also losing a bit of his humanity in the process - although he was an antagonist throughout the movie it is worth noting that Janosz was never a truly evil individual, just very misguided and desperate - thus easily manipulated by more sinister beings such as Vigo.
After gaining a link to Vigo his character darkened considerably and he would go as far as utilizing demonic powers to kidnap Oscar and take him to be sacrificed to Vigo, so that the evil spirit could live on in Oscar's body: Janosz was promised that in exchange for his betrayal he would be given Dana as his bride and a status as an adopted "father" to the reborn Vigo.
Janosz was ultimately defeated by the Ghostbusters using a special device that reversed Vigo's "mood slime" via transforming the negative psychic energy into positive ones - this was really the only non-lethal way to deal with Janosz (since he was human) and worked very well as he became almost instantly reformed afterwards - his ultimate fate is unknown but it is safe to assume he resumed his role as a supervisor at the Manhattan Museum of Art.
Janosz was originally a neutral character who while mostly harmless was also quite antagonistic in his own way, having a crush on Dana and easily becoming jealous - he was, however, not a threat to anyone and largely seemed your average unwanted love interest.
Yet when Vigo manifested things changed quickly and Janosz was quick to throw himself at the feet of the evil spirit, originally out of sheer fear and awe - however, Janosz soon grew darker as Vigo convinced him that he could woe Dana and become a future "father" to a "god" by helping him capture Oscar : his fear quickly fading into a dark obsession he willingly allowed himself to be further corrupted by Vigo and thus his neutral status changed to that of a malevolent stalker who was willing to take Dana by force, as well as put Oscar in great danger just to fulfil his own selfish ambitions.
Yet even in his darkest moments Janosz was still a rather comedic villain and was not terribly threatening, indeed he was cowardly and weak - though he had moments of being a more serious threat, such as when he kidnapped Oscar in the guise of a ghostly "nanny".
After Vigo's defeat and being sprayed with positive mood slime Janosz personality improved dramatically as his evil urges seemed to vanish, given his nature it is assumed Janosz would return to his neutral status after these events and no longer threatened anyone.
Janosz originally had no unusual powers, indeed he was physically weak and cowardly in nature, making him a rather comedic and harmless figure - after willingly giving himself up as an enforcer of Vigo he would be granted a darker personality and a collection of demonic powers, sufficiently powerful that he could turn himself into a ghostly creature in order to capture Oscar (his magic was sufficient that he was mistaken as a ghost by one of the main cast, though Dana knew it was Janosz).
Even with his new powers Janosz didn't utilize them to their full potential and tried to avoid physical confrontation, still being rather cowardly in nature - after being exposed to positive mood slime all his demonic powers were removed, as were his more destructive urges.. making him a non-powered and more mentally stable human being once more.