Full Name: Elizabeth Madison
Appearances: Issues #17, 23, 28, 31-34, 36.
Jenny wrote "I have no idea what's going on in that head" when describing Elizabeth in her diary.
Readers don't know much more than Jenny does, but if issue #36 is any indication, we can conclude that Elizabeth's parents are not on the best of terms, nor has she displayed much affection for her brother Spike (and naturally, the feeling is mutual).
We know that Elizabeth spends a lot of time reading; we know that her family lives in a large expensive house; and we know that she keeps her feelings to herself in school.
Of the stories she's appeared in to date, Elizabeth's dialogue has yielded an average of exactly seven words per issue.