Ok. Before I tell you how to get a ghost buddy I will tell you what it does. When you talk to someone every now and then the ghost wil take some sanity away from the person you are talking to.

To get the ghost you need to go to the store when you are very, very, very far into the game. Buy the cute ghost painting. Go to you suite. Hang the painting on the wall. Admire it and... VOILA!! you have a ghost buddy.

By the way the ghost buddy does not follow you around and will not be on screen all the time. He will just appear every so often. And as long as you keep the the painting up you keep ghost buddy. But... should you take it down he will disapear...