Episode S02E05 of Game of Thrones is titled "The Ghost of Harrenhal". But I wonder to whom this title actually refers. Does this refer to Arya, seeing that she is more or less incognito as Tywin Lannister's handmaiden. Or does it refer to "a man" she befriends and who mysteriously kills people in the castle on her command? Or is this maybe deliberately left ambiguous? Who is the Ghost of Harrenhal?
A little history first.
These days Harrenhal is considered an ominous location with an eerie air about it. The place is said to be haunted by its founder Harren the Black and his descendents. Therefore the phrase: The Ghost of Harrenhall.
Taking a page from the book A Clash of Kings, I would say that the episode refers to Arya as the Ghost of Harrenhal. In the books, all the occurrences at Harrenhal are told from Arya's POV. However, in the show it is a mix of her and Tywin's narration.
After Arya, Gendry and Hot-Pie have been taken hostage at Harrenhal by the Lannisters, Arya devotes a lot of energy into being as inconspicuous as possible. At times she even compares herself to the rats of Harrenhal, omnipresent yet hidden in plain sight. She attributes these characteristics to herself and knowing that she is the reason behind the strange deaths there, names herself the Ghost of Harrenhal.
I don't recall any mention of the ghosts in Harrenhal in the TV show.
In the books however, it's a common belief the Harrenhal is haunted, for example from the 4th and 7th Arya chapters in A Clash of Kings:
But of course, she believes none of it and explains away the stories in her 7th chapter:
But at the end of the chapter, after she tells Jaqen H'ghar to kill Chiswyck (instead of The Tickler, as in the TV show), she reveals who the title refers to (emphasis mine):