Who is the ghost in this novel?

The ghost in the story is Sir Simon of Canterville Hall. When he was alive in the sixteenth century, Sir Simon killed his wife. As a result of this crime, he has been forced to stay on the earth haunting Canterville Hall, his soul not at rest.
Sir Simon takes his job as a ghost very seriously, believing it is his mission to thoroughly frighten the inhabitants of Canterville Hall. Therefore, he is frustrated when the American Otis family rents the hall and refuses to be intimidated by him. In fact, they end up frightening him more than he frightens them.
As we discover, however, Sir Simon more than anything else wants to stop existing as a ghost. He wants to die and find the peace that has eluded him.