Does Jill only appear in The Silver Chair and The Last Battle? Is she related to the Pevensie children? Seems she is just a classmate of Eustace, who is relative of the Penevsies. Diggory is related too, but Polly is not, she's a neighbor of Diggory. What I should do is go back and read the books, but I teach English and have other texts to read, so I can't take the time to do this very often.
I think it's funny, too, that Eustace calls her "Pole" in the early parts. This is probably because of the modern school they attend, but it's cute.
