The McPhearson Phantom is the ghost of a woman whose husband was thrown to The Loch Ness Monster by Ma's great-great-aunt. The ghost thought it was Muriel's great-great-aunt who committed the crime. She then decided to break up Muriel and Eustace's marriage by playing several tricks on Eustace. Ma (posing as a therapist) wanted to break up their marriage also. The two started working together, until it was revealed that it was Ma's great-great-aunt who threw her husband into the Loch Ness Monster, not Muriel's. The Phantom then stoped trying to ruin the Bagges' marriage, and decided to have revenge on Ma.
Her black ghost appearance resembles Oogie Boogies Shadow