Greta Hansen, also known as Madame Greta and "The Manipulator", is an evil old woman who lives in a haunted house and a character in the Season 5 episode "Old Sins Cast Long Shadows" of The Ghost Whisperer.
She was killed when her wheelchair feels down the stairs in the early 1940's. When she died, the Shadows told her to give them children. In exchange, they wouldn't feed on Greta.
Greta is a manipulator. She told innocent young Cassidy that crossing over into the light is a bad thing and told her she would be punished if she told anyone. Melinda saw a vision of the past where she lies to the parents of Naomi telling her that she was going into the light when in reality, she lied to them. In the hospital, she tricks a sick little boy into coming to her house to play with Cassidy. Greta then threatened Melinda's son Aiden by saying "I always know where your boy is. Don't you?" Melinda answered back by saying, "I won't let you threaten my son." Greta answered back by saying, "Then stay out of my business."
Melinda ignored her and convinced Cassidy to go into the light. Before Cassidy could, Madame Greta pulls her and her friends back inside the house. She tried to convince Cassidy that she was too naughty to go into the light. She said that Cassidy tricked those ghost kids so she could have friends. Greta gave Melinda one final warning to get out of her house before she took matters into her own hands. Eventually, Cassidy and her friends escape into the light while Madame Greta is captured by the Shadows.
According to Ned, Harry Houdini even attended one of her seances but couldn't debunk if Greta was legitimate with her work or not.