A close-up of Cueball from 1063: Kill Hitler
|First appearance||12: Poisson|
Cueball is a stick figure character in xkcd. Cueball is distinguished from other characters by having no distinguishing features (including no hair or hat). Cueball is the most commonly depicted xkcd character.
Unlike most other xkcd characters, Cueball doesn't necessarily represent the same character from comic to comic, and is not necessarily a unique character in any given strip. Instead, he represents a generic everyman. In several comics (1089: Internal Monologue, for example) there are multiple such stick figures, any of whom could be called Cueball.
Early xkcd comics often feature a Cueball-style character who occasionally included facial features. Randall appears not to have standardized his character lineup early in the comic's run, and as a result, early characters are sometimes difficult to identify as Cueball or not.
The character is never actually referred to as Cueball in xkcd. The name was developed by early contributions to explain xkcd, and has stuck. It refers to the visual similarity of Cueball's head to a billiard ball. In some strips a cueball character named Rob appears.
We do know for a fact that his name is not Terry, check the transcript.