In "Fat Camp", he was a drug addict who was supposed to be in rehab. He had somehow escaped, though, and got 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 from Stan and Kyle. He can impersonate voices with ease, allowing him to impersonate Cartman easily. Undisguised, his voice is a lot less harsh than Cartman's voice, reminiscent of Clyde's voice when he speaks in a low tone.
Eventually, Kyle exposes 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). Unfortunately, the extreme pressure inside Crabtree killed the impostor.
His actual appearance is unknown since he has only been seen disguised as someone else. Originally he wore Cartman's blue hat, red coat, and yellow gloves. After Kyle exposed him, he forced him to stand in for Kenny's final stunt on his show, so he wore Kenny's orange parka. Underneath, though, he has auburn hair, similar somewhat to Kyle's, but his hair sticks straight up, unlike Kyle's "Jewfro".
It was revealed in "Fat Camp" that he was a drug addict who escaped rehab. Using these facts, we can assume he doesn't even care about his health. Later in the episode, he attempted to start an "I learned something today..." speech, however, whether he was doing this because he actually cared or whether he did it to simply keep up the masquerade is unknown.