The Copycat  is a shape shifting entity, also called a "metamorph," that has the ability to change into any form it wants.  He was haunting Mrs. Campbell's upper class apartment room, but made it past the Ghostbusters because he doesn't give off P.K.E. readings while changing his form.
The Copycat first appeared in the home of Miss Campbell and displayed a mischievous streak. It broke her fine china, rearranged the furniture, scared her dog Pieface, and left the living room in shambles. After the Ghostbusters failed to find him, the ghost hitched a ride on Ecto-1 in the form of the No-Ghost sign. After the Ghostbusters returned to the Firehouse, it assumed many forms and pulled several cruel pranks on the Ghostbusters, framing each other in the process. Eventually, they realized what was going on and (after activating a security system to keep it from getting out of the Firehouse) started searching for it. But even then, the Copycat still had the upper hand, escaping every time they found it, and even managing to beat them by taking on the forms of Captain Steel and a Proton Stream.
The only way it could be trapped was if he was in a ghost form,  and since it wasn't known what the Copycat's true form was (if it even had one), Slimer tricked the Copycat into assuming his form. The Ghostbusters hesitated at first because they were not sure which one was the real Slimer and which one was the Copycat. Egon Spengler was sure that Slimer was the one on the right, but Peter Venkman made sure by showing Slimer his favorite candy bar. After Slimer flew out of the line of fire to eat the candy bar, the Ghostbusters zapped the Copycat and caught him in a Trap before he could morph. It is unknown if the Copycat is stuck in the form of Slimer or if he continues his copycat pranks by mimicking the forms of the ghosts inside the Containment Unit.
The Copycat has the ability to shape shift to anything it sees from people to ghosts to inanimate objects.  It is also capable of duplicating the abilities of whoever or whatever it copies, such as when it possessed the superpowers of Captain Steel, the hero it impersonated or when it became a duplicate of a Proton Stream. Because of its shapeshifting abilities, it doesn't give off any P.K.E. readings, thus rendering the P.K.E. Meter useless and making it one of the most difficult ghosts for the Ghostbusters to find, let alone capture. Due to its playful nature,  it's known for pulling pranks on unsuspected people, including the Ghostbusters. It is capable of talking and has a trademark cackling laugh. One of its lines are "Who you gonna call? Hahahahaha."
Also since it's a Metamorph, it has no true ghost form as far as it has been revealed. Therefore, it can't be trapped unless it's in a ghost form.
Copycat takes a total of 18 forms.
- First appeared in Miss Campbell's kitchen as a cooked turkey.
- Scared Miss Campbell's poodle by morphing into it.
- Hitches a ride on Ecto-1 as the No Ghost Symbol on the rear door.
- Deceived Slimer by appearing to be a giant hamburger, only to run off before Slimer could take a bite.
- As Peter Venkman, it saw Winston Zeddemore doing work on Ecto-1. It then disconnected a tube, making oil spill on Winston.
- Assumed the form of a lamp so that the real Peter Venkman would take the rap.
- Morphs into Janine and learns about their Transdimensional Portal experiment.
- Hides outside the lab as a flower, vase, and table.
- In the form of Egon Spengler, shuts off the Transdimensional Portal's power.
- Morphs into a sofa in the living room area.
- Morphs into cheese in one of the lower cabinets of the kitchen.
- Morphs into a brain in a jar in Egon's lab.
- Turned into a limited edition Captain Steel comic.
- Morphed into Captain Steel
- Disguised as Peter's Hula Girl Lamp
- After being shot at by a proton beam, it morphs into it and zigzags around the room, forcing the Ghostbusters to duck, and chasing Slimer into a water cooler.
- Assumed the form of a foot stool
- Changed into Slimer
- On page five of Ghostbusters Issue #6, several Copycat, in burger form, dolls can be see in the bottom row of the Fantastic Land stand.
- On the subscription cover of Ghostbusters Volume 2 Issue #18, Copycat makes a non-canon cameo.
- ↑ Janine Melnitz (2009). The Real Ghostbusters - "The Copycat" (1987) (DVD ts. 13:03-13:06). Time Life Entertainment. Janine says: "Take it easy, Slimer. Copycat can't hurt you."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Copycat" (1987) (DVD ts. 11:30-11:31). Time Life Entertainment. Egon says: "It must be a metamorph!"
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Copycat" (1987) (DVD ts. 20:07-20:10). Time Life Entertainment. Ray says: "The copycat has to turn into a ghost before we can trap it, understand?"
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Copycat" (1987) (DVD ts. 11:35-11:39). Time Life Entertainment. Ray says: "That thing is a shape changer. A copycat that can copy anything it sees."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Copycat" (1987) (DVD ts. 19:56-19:57). Time Life Entertainment. Ray says: "Metamorphs are playful, right?"