In xkcd, there is a cast of recurring characters which are often the focus of comic strips. The characters are created by Randall Munroe. The characters xkcd are stick figures, typically with distinguishing features.
The most frequently appearing characters in xkcd are Cueball and Megan, who act as the everyman and everywoman. They tend to be the go-to characters when no specific personality traits are desired for the comic. Other characters with more distinctive personalities have been created and are used where appropriate.
Randall Munroe does not appear to have had a fixed cast of characters in mind when he first created xkcd. The first comic features a non-stick figure character in a style no longer used in the comic today. The next four comics do not feature characters at all. The first use of stick figures was in the sixth comic, 6: Irony in which two basic stick figures are depicted (either of which would qualify as a Cueball figure).
Beginning in 12: Poisson, Randall began using stick figures fairly commonly in xkcd strips. "Poisson" features the first clearly recurring character, Black Hat. He recurs for the first time in 29: Hitler, in which there is clear reference in the title-text to "the hat guy". This is the first indication we have that Randall had begun contemplating recurring characters (although it is not conclusively clear that the hatted characters in "Poisson" and "Hitler" were intended to be the same character.)
Throughout the early period, random non-recurring characters continue to be used including ones with facial features, and characters with various hairstyles which are no longer seen.