There are several characters in Senta and the Steel Dragon Book 0: Brechalon that appear entirely or mostly in flashbacks.
Smedley Bassington: Wizard Bassington appears mostly in flashbacks though he appears in the present in one chapter. Bassington is an important character in the series and plays a large part in Books 3 and 5. He is a paramour of Zurfina and he is a government wizard. He is also, as we find out, a great manipulator. I had a very clear image of what Bassington looked like. His appearance is based on Stephen King. I set up my copy of the Dark Tower to the author's photograph whenever I was writing him. I had been carrying the name Smedley around for a while, and it just seemed to fit. Except for his final scene in this book, he's not that interesting, I don't think. He has his part to play, and plays it. I really enjoyed writing him in Book 3: The Drache Girl. He is a great foil for Senta.
Master Akalos: Master Akalos was the Dechantagne tutor, a cowardly little man, and not very important to the story. He is referenced once or twice, but Brechalon is the only place we see him, and that in a flashback.
Iphegenia Dechantagne: The mother of Terrence, Iolanthe, and Augie is murdered by her husband, as we see in a flashback in Book 0. I don't think she is ever mentioned again, but she plays an important role nonetheless, in shaping the Dechantagne siblings. I love the name Iphegenia, and have always been fascinated by the myth of Agamemnon's sacrificing her. I would have used the name somewhere, but it fits here.
Lucius Dechantagne: The father of the Dechantagne's, he is only spoken of in the later books, having died some time before Brechalon, and having lived without facing real punishment for his wife's murder, though it does apparently cost him his standing in society.
Jolon Bendrin: As we find out in book 0 flashbacks, Jolon Bendrin is a rapist. He is mentioned only briefly in book 1, but finally makes a satisfying appearance in the flesh in book 5.
Simon Mudgett: The man who was in bed with Iphegenia Dechantagne when her husband kills her. He is mentioned once more in the series. I pulled Simon out of nowhere, but Mudgett I have been saving for a while. The name comes from Herman Webster Mudgett (aka H. H. Holmes), America's first serial killer.