Peeves Not A Real Ghost?

[Karen]

No: HPFGUIDX 47048

I am re-reading SS/PS and I came across something curious on pf. 
115.  This is what it said:
	
"My dear Friar, haven't we given Peeves all the chances he deserves?  
He gives us all a bad name 	and you know, he's not really even a 
ghost- I say, what are you all doing here?"

Sir Nicholas is the one speaking in this quote.  What does he mean 
when he says Peeves isn't really a ghost?  Was there something in the 
other three books that I forgot telling why is isn`t a real ghost?   
What does everyone think of this?

Karen