Lonely nerds who tend to read real dictionaries for fun. Use urban dictionary to slag off their roommates/lab partners/their only friend, e.g. "John - definition: a big gay faggot who smells like cottage cheese"
The lucky ones will be part of a pack of two or three irrelevant cock munchers, surfing urban dictionary in their spare time, making up shyte phrases, e.g. "shralp", "a3 ayri" and "battlebot".
The urban dictionary definer sits on a bench on his own, wearing a sweater vest and eating peanut butter from the tub with his finger. Often carries a laptop around just in case a new phrase like "kkthnx" pops into mind. He is the prey of many predators including jocks, cheerleaders, dead plants and rocks.
"Hey sniperdude 321223 im urban dictionary defining at the moment, ill play halo with you later though"
"ok no worries. speak to you soon halospartan 25924495"
"hey look its an urban dictionary definer..."
"how can you tell?"
"he is shit at life."