Billy Miller is a child from South Park presumably in 4th grade.
Billy Miller appears only in "Medicinal Fried Chicken", in which he appears as the KFC trafficking lord (a play on drug lord) and takes his business very seriously. He hires Cartman so that he can repay his debt for the popcorn chicken he ate. Soon after sending him to set up the deal with Colonel Sanders, he is overthrown by Cartman. He seems to have survived the shoot out, but since we did not see him afterwords it is possible he could have been killed.
Billy has a very businesslike personality and seems to understand what it means to be trafficking chicken. He seems to prefer solving problems such as debt in a business like way "You're a big boy Eric, maybe you can pay me back another way." And is shocked by Cartman's ridiculous order of 10,000 boxes of chicken. He releases that he "can't move that kind of chicken." And acts like a stereotypical drug lord. " I'm not making any money here, I'm losing it! I have to pay for more security, I got the cops up my ass and I have to hire these sorters to get the chicken ready for distribution!" Billy can be noted as replacing Loogie as South Park's young crime kingpin.