The Beer Ghost, introduced in The Scariest Ghost Ever, is a small ghost that steals and drinks other people's beers. His favorite victim seems to be Denmark who is horrified by the thought of someone stealing his beer, let alone a ghost. Even when Denmark seemingly thwarts the ghost's plans, the Beer Ghost is still able to get his beer by doing the Kiss of Beer like in No Beer Left. The Beer Ghost has also stolen Finland's beer and was unaffected by Finland's knife. The Beer Ghost has demonstrated that it can speak in it's first named appearance (see quote), yet usually emits a sound written as a variant of "Uuuuuh".
The Beer Ghost is notable for being the only recurring character that is not a representation of a country, excluding Ottawa, and is the only spirit that has the ability to speak in the series. He's also notable for being able to catch the flu in spite of being a ghost.
Outside of the comics, the Beer Ghost was part of a contest in 2011 that had a grand prize of two London Comic Con Tickets, which required entrants to print out a picture of the Beer Ghost and place it in a creative spot to take a photo.
The Beer Ghost was also made into a label badge which is sold on the site's store, Beer Ghost Badge, by itself or together with the Scandinavia and the World poster, which has a picture of the Beer Ghost on Denmark's head: Poster and Badge