Jacob Marley was one of the ghosts in The Real Ghostbusters in episode "X-Mas Marks the Spot". He made a very brief appearance in the episode, so the least is known about him. Unlike the Ghost of Christmas Past, the Ghost of Christmas Present, and the Ghost of Christmas Future, Marley managed to avoid capture by the Ghostbusters and was only detected by the PKE meter.
Personality
While not much is said about the ghost, as he only howls as he leaves Ebenezer Scrooge's place, it is known in the novel A Christmas Carol that Marley was Ebenezer Scrooge's partner who had died seven years ago. He came to Scrooge on Christmas Eve to tell him that he was suffering because of the sins he commited in life and that he would suffer the same fate if he wouldn't let three ghosts show him the error of his ways. His chains were meant to symbolize his suffering. After Egon freed the three ghosts, he prepared Scrooge for their arrival by giving Jacob's warning, telling Scrooge he would be visited by the three ghosts.