A Chameleonian is an alien that can perfectly mimic the appearance of any creature or object around it. These aliens are such great masters of disguise that no one knows what a Chameleonian actually looks like, not even the alien itself.
Chameleonians are not only indistinguishable by appearance but in identity as well. This causes them to undergo identity crises, making them question whether they are the real person or not.
The Chameleonian impersonates Dr. Gel at the end of the episode, quickly driving him insane as he struggles to understand who the true Doctor is and who is the impostor.
The finale reveals the Chameleonian was actually The Narrator.