Who is Count Olaf/Stephano from The Reptile Room and what is their importance?

Count Olaf is described as a tall, thin man, with shiny eyes and a unibrow. He has a tattoo of an eye on his left ankle. In The Reptile Room, Count Olaf is disguised as Stephano, Uncle Monty's new assistant. He has shaved his eyebrows and his head, and he has grown a very long beard. The tattoo, although first visible to the children, is disguised with makeup until the end of the book. Count Olaf is the novel's antagonist.