Muggles' Guide to Harry Potter/Characters/The Bloody Baron

Overview
The Bloody Baron is the Slytherin House ghost at Hogwarts. He has "blank staring eyes, a gaunt face, and robes stained with silver blood." No one that we know of has ever asked how he became covered in blood. He is the only one that Peeves is afraid of. His favourite pastime is clanking around in the Astronomy Tower.

Philosopher's Stone
At the Welcoming Feast, Harry sees the Bloody Baron at the Slytherin table. He is apparently sitting quite near Draco Malfoy, who is looking rather uncomfortable; this cheers Harry up a bit. Seamus asks Nearly Headless Nick why he is all covered in blood, and Nick replies, primly, that he had never asked.

We hear later that Professor Dumbledore and the Bloody Baron are the only people who have any measure of control over Peeves.

When Harry, Ron, and Hermione are on their way to the Trophy Room for the Wizard's Duel with Draco, they run into Neville who has been locked out of Gryffindor Tower because he forgot the password. Neville insists on going with them; he says the Bloody Baron has been by twice already.

Harry impersonates the Bloody Baron while under his Invisibility Cloak with Ron and Hermione, to avoid being caught by Peeves as they head for the trap door.

Half-Blood Prince
Returning from Hagrid's hut, where he has just retrieved a much-needed memory from Horace Slughorn, Harry finds his way back into the Common Room barred by The Fat Lady who tells him that it's after midnight and the password has changed. Harry comments to Nearly Headless Nick, who is passing idly through the wall, that he really wants to see Dumbledore; Nick tells him that the Bloody Baron had just seen him returning.

Deathly Hallows
From The Grey Lady, we learn that the Bloody Baron had been sent to bring her back from the forest in Albania where she had been hiding, to see her mother Rowena Ravenclaw on her deathbed. When she refused, he became angry, and killed her in a fit of rage. Discovering what he had done, he was instantly horrified and guilty and took his own life by turning his dagger on himself. The Baron has since been covered in her blood and bound himself with heavy chains as punishment for the heinous crime he committed against the woman he loved.

Strengths
He controls Peeves, as Peeves fears him.

Weaknesses
He is dead

Relationships with Other Characters
While we never talk to the Baron directly, it is revealed that in life he was greatly in love with Helena Ravenclaw who became the Grey Lady.