I think all my character discussions for Senta and the Steel Dragon will have to carry a spoiler alert. So much happens to all of them over the course of the series that, though I try not to, I might inadvertantly spill some secrets about one or two of them. So, read this at your own risk.
Brechalon is a prequel. I wrote it long after I had written books 1, 3, and 5, so it provides a look at some of the events of the past that are only hinted at in the other books. I don't know whether to recommend that you read it first or last.
Terrence Dechantagne is one of the major characters of the series. As he appears in Brechalon, he is in the army, serving in the cavalry. During the course of the story, he takes a furlough to spend time at home. Terrence is a drug addict, addicted to the "See Spice," white opthalium, a magical drug that transports him to a world where his troubles are all soothed away. The ultimate cause of this addiction is his eyewitnessing the murder of his mother by his father when he was twelve. Add to this, the generally unpleasant life that he has led since.
Terrence is a thoughtful man, a collector of rare books, but has been thrust into the position of a man of action and violence by the expectations of others. His general self-loathing extends to anyone who cares about him. In book 0, he is at his low point, spending most of his time under the influence.