Ghostbusters

Parapsychologists Peter Venkman, Raymond Stantz, and Egon Spengler are called to the New York Public Library to investigate recent paranormal activity. They encounter the ghost of a dead librarian but are frightened away when she transforms into a horrifying monster. After losing their jobs at Columbia University, the trio establish a paranormal investigation and elimination service known as "Ghostbusters". They develop high-tech equipment capable of capturing ghosts and open their business in a disused, run-down firehouse. Egon warns them never to cross the energy streams of their proton pack weapons, as this could cause a catastrophic explosion. They capture their first ghost, Slimer, at a hotel and deposit it in a specially built containment unit in the firehouse basement. As paranormal activity increases in New York City, they hire a fourth member, Winston Zeddemore, to cope with demand.
The Ghostbusters are retained by cellist Dana Barrett, whose apartment is haunted by a demonic spirit, Zuul, a demigod worshiped as a servant to Gozer the Gozerian, a Sumerian shape-shifting god of destruction. Venkman takes a particular interest in the case, and competes with Dana's neighbor, accountant Louis Tully, for her affections. As the Ghostbusters investigate, Dana is demonically possessed by Zuul, which declares itself the "Gatekeeper", and Louis by a similar demon, Vinz Clortho, the "Keymaster". Both demons speak of the coming of the destructive Gozer and the release of the imprisoned ghosts, and the Ghostbusters take steps to keep the two apart.
Walter Peck (William Atherton), a lawyer representing the Environmental Protection Agency, has the Ghostbusters arrested for operating unlicensed waste handlers and orders their ghost containment system deactivated, causing an explosion that releases hundreds of ghosts. The ghosts wreak havoc throughout the city while Louis/Vinz advances toward Dana/Zuul's apartment. Their romantic encounter opens the gate and transforms them into supernatural hounds. Consulting blueprints of Dana's apartment building, the Ghostbusters learn that mad doctor and cult leader Ivo Shandor, claiming humanity was too sick to survive after World War I, designed the building as a gateway to summon Gozer and bring about the end of the world.
The Ghostbusters are released from custody to combat the supernatural crisis, but after reaching the roof of Dana's building, they are unable to prevent the arrival of Gozer, who appears in the form of a woman. Briefly subdued by the team, Gozer disappears, but her voice echoes that the "destructor" will follow, taking a form chosen by the team. Ray inadvertently recalls a beloved corporate mascot from his childhoodâ"something that could never, ever possibly destroy us"â and the destructor arrives in the form of a giant Stay Puft Marshmallow Man and attacks the city. The Ghostbusters cross their proton pack energy streams (reversing the particle flow) and fire them against Gozer's portal; the explosion defeats the Stay Puft Marshmallow Man and frees Dana and Louis. As thousands of New Yorkers wipe themselves free of marshmallow, the Ghostbusters are welcomed on the street as heroes.

Question: Who haunts Dana Barrett's apartment?
Answer: Zul