Little is known about the past or identity of Mr. Chameleon. When Ben Jackson and Tommy Frazer first arrive in Grayworld, Mr. Chameleon is described as an evil photographer who was assigned to take a class photo for the students of Bell Valley School in 1947.
The students all hated him, and while getting their picture taken, messed around. This infuriated Mr. Chameleon, who snapped their picture in a single bright flash. The students were then transported to Grayworld — a strange black and white dimension on the other side of the School walls. The vanishing of the students would lead to the old school being boarded up, and statues of the missing class being put up in their memory. A majority of the class would be driven insane after being trapped, and are permanently stuck as the age they were when the picture was taken.
When Ben and Tommy eventually escape Grayworld with the help of Thalia, they discover a very much alive Mr. Chameleon about to take a new class photo. However, he snaps the picture while Tommy screams, sending a new class to Grayworld.
Mr Chameleon's physical appearance is never described in the book. In Goosebumps HorrorTown he's also revealed to have grey skin like the Greyworld kids and has bright red eyes. He wears a black suit and hat with glasses and carries a cane.
Mr. Chameleon hates children, although it is never revealed as to why exactly. He seems to have a short temper around them, judging by how fast he sent the kids of 1947 to Grayworld. However, it is very likely he did this for his own amusement, as he sends a new class to Grayworld.
It can be assumed that either Mr. Chameleon or his camera have some kind of supernatural ability.
- Mr. Chameleon is similar to Dr. Fritz Fredericks in that both men are owners (and quite possibly both men are creators as well) of evil cameras. The main difference being that Fredericks' camera creates dark futures for whoever's picture is taken, while Chameleon's camera sends children into Grayworld.
- Before his appearance in Goosebumps HorrorTown, he's foreshadowed at the end of the A Shocker on Shock Street event.
- Goosebumps HorrorTown reveals that his first name is Mumler. This is a reference to William Mumler, the inventor of spirit photography.