Cartman Impostor was a young drug addict. He appeared in "Fat Camp".
His general appearence is unknown, since he is always disguised as someone else. Originally he wore Cartman's blue hat, red coat and yellow gloves. When Kyle found him out, though, he forced him to stand in for Kenny's final stunt on his show, so he wore Kenny's orange clothes. Underneath, though, he has auburn hair, similar somewhat to Kyle's, but his hair sticks straight up instead of Kyle's "Jewfro" style.
His voice is very similar to Cartman's - he could impersonate Cartman with relative ease. However, undisguised it is a lot less harsh than Cartman's voice, reminiscient of [Clyde's voice when he speaks in a low tone. Going by his height, he is a 3rd or 4th grader.
In Fat Camp, the Impostor was a drug addict who was supposed to be in rehab for it. He had somehow escaped, though, and got by by supplying Cartman, who was in fat camp, with candy in return for a cut of the profits - originally 10%, but eventually increased to 20% since he was also impersonating Cartman, generating suspicion in Stan and Kyle. Eventually Kyle rumbles him after "Cartman" delivers his first decent "I learned something today" speech - something the real Cartman would never do. Since Kenny had been arrested for prostitution in New York, Kyle forced the impostor to stand in for Kenny's last stunt (staying in Ms. Crabtree's uterus for 6 hours) since with his hood up nobody could tell the difference. Unfortunately, the extreme pressure inside Crabtree killed the impostor.