There are quite a few characters who do not appear in the story of Senta and the Steel Dragon, but are only referenced. Here are three big ones:
Magnus the Great: Magnus was king of the Zur two thousand years before the events in the story. He was a conqueror who carried on his father's conquest of the continent of Sumir, more or less unifying the culture of mankind. He occupies a place in history much like our own Julius Caesar or Alexander the Great. At the end of his reign, his empire fell apart, partially due to the antics of his daughter Zurfina, though no one has described the exact details. Zurfina the sorceress is the namesake of Magnus's daughter.
Kafira Kristos: Probably the most important minor character in the book is Kafira Kristos. She occupies the place in the world of Senta and the Steel Dragon that Jesus Christ does in ours. Her life and martydom just after the time of Magnus the Great creates the divide between the two religions in the story--Kafirites and Zaeri.
The idea for Kafira came from a theological paper I once read. It wondered, assuming that life existed on countless planets of the universe, would Jesus have lived and died on each one of them, or would they have had their own savior. I decided that for the story, this world similar but not quite our own, would have its own, and further decided that she would be female.
Kafira is also the basis for a great deal of blasphemous cussing in the story. Kafira! Kafira Kriston! Kafira's Cross! Kafira's Tits! Kafira in a Hand Basket! and worst of all, Kafira's Bloody Twat!
Kazia Garstone: There are many books and writers referenced in the story, because several characters are either writers, devoted readers, or book collectors. I have a whole list of authors and their works, some of whom I never used. Kazia Garstone is referenced more than any of them. She was a muck-raker as well as an author and is considered quite scandelous in polite society. Many consider her a socialist, but her books are widely read and early editions are very valuable.