Imaginary Friends and Enemies - S4-E6
Continuity mistake: In the scene in the woods where we first see Owen's ghost, as the camera cuts back and forth between Melinda talking to Owen and what her friend sees, the two women are alternately closer and then significantly farther apart. They're supposed to be standing basically in the same spots, only taking maybe one or two small steps during Melinda's conversation with Owen.
Answer: This "spirit" appears in several episodes throughout the rest of the season including the season finale. He's a "bad spirit" that is intent on keeping souls trapped on Earth. With the death of Andrea (her business partner) he wanted to make a deal with Melinda, to give him HER soul to release everyone else including Andrea to cross over.
Boobra