The scary prisoner was one of the men incarcerated in the Chin Village prison. He had a strong build, piercings in both ears, and his torso had a large snake tattoo that covered the top of his chest and slithered down his right arm.
The scary prisoner met Aang when the latter was imprisoned by the townsfolk of Chin Village for the murder of their hero, Chin the Great, and had to wait for Katara and Sokka to return from Kyoshi Island with evidence of his innocence. The prisoner emerged from the shadows and commented that, since Aang had a bald head and "nice" tattoos, he would fit in really well with the other prisoners. He charged at Aang, but was restrained by the chain around his neck.
As the day went on, the scary prisoner, Aang, and some other prisoners were found sitting in a circle, talking about Aang's love trouble with Katara. His tone was very gentle when he assured Aang that Katara would come around if he would just "hang in there". When Aang doubted this, the scary prisoner responded that Aang was "smart, handsome, funny, not to mention [he was] the Avatar".