Following his death, the spirit of Emilio's Wife's Lover remained on earth and unable to move on, due to unresolved issues. During Dr. Zorba's travels around the world, he met and captured Emilio, his wife and her lover and imprisoned them in his mansion.
In the mansion, the lover is shown several times to be kissing Emilio's Wife, only for Emilio to then catch them and kill his wife with his meat cleaver; suggesting that the ghosts of Emilio, his wife and the lover are reliving the moment when Emilio discovered the affair over and over.
When Dr. Zorba was murdered and became the twelfth ghost, Cyrus Zorba and his family moved into the mansion.
The Lover and the other eleven ghosts presumably crossed over when Benjamin Rush died and became the thirteenth ghost.