There are a few other characters who appear in The Dark and Forbidding Land.

Professor Calliere: Calliere is Iolanthe’s husband and plays a bigger part in books 1 and 3, but he does play his part here as well.

Lon Fonstan: Lon is one of those guys who appears on maybe a page in each of the books, but he’s there and he has some fun interactions with others, especially Senta.  He really shines in book 3.

Mr. Brockton: Brockton is a wizard who appears in books 2 and 4.  He works in the War Ministry with Wizard Bassington, but isn’t nearly as powerful.

Miss Gertz: Miss Gertz is the mayor’s secretary.

Mrs. Godwin: An old household servant of the Dechantagnes, Mrs. Godwin lives in the family house.

Paxton Brown: Brown makes his appearance in The Dark and Forbidding Land, but doesn’t really become an important character until book 5.