My thoughts on this are that the oldest ghost to die in the Harry Potter books would have been the Grey Lady (Helena Ravenclaw). She is the daughter of Rowena Ravenclaw, which is what leads me to believe that, out of the ghosts we've encountered in the books, she would be the oldest.
The ghost of the Slytherin house in Harry Potter is the Bloody Baron.
Moaning Myrtle died long before Harry Potter was even born, she was a ghost when Harry first met her.
I think there's a ghost for each house plus one so about five
Professor Binns is a ghost.
He is a ghost.
because its happy.
Sir Nicholas ("Nearly Headless Nick") appears in most of the Harry Potter books.Moaning Myrtle in HP2And Peeves, but he is a poltergeist, not a ghost.
Cuthbert was the first name of Professor Binns, the ghost that taught History of Magic at Hogwarts.
Moaning Myrtle is the ghost that lives in the Girl's toilet.
I believe that is Nearly Headless Nick.
If you're referring to his father's ghost he first saw him in the graveyard where Voldemort's father was buried.
The Slytherin house ghost is the Bloody Baron.
Professor Binns who taught History of Magic.
you can't, she's a ghost
Cuthbert Binns, who teaches History of Magic.
Well, ghosts in Harry Potter's world are able to write (letters to other ghosts, for example), but I don't think it is what you meant.
No he did appear to Harry Potter in a ghost type form when Harry was in limbo though.
The ghost's name is Moaning Myrtle. She is played by the actress Shirley Henderson.
No, he is only shown as a ghost in The Goblet of Fire.
They are dementors.
Harry Potter: The Game Series nobody care for.
Not everyone becomes a ghost. And for this reason not all of those that have died in Harry Potter can be seen as ghosts. In fact none of the characters that the reader sees die actually become ghosts.
If by ghost girl you mean Moaning Myrtle, the ghost who haunts the girls bathroom, she was killed when she looked into the Basilisk's eyes.