Transpecial has three major characters. Suza was, perhaps, the easy one - or perhaps not.
Warren Taylor is old and cynical. He was in Earth intelligence during the Mars War, but retired afterwards. Since then, he's been an anthropological researcher, that being what his original training was in.
He's a tough guy, but more than a little xenophobic. Humanity comes first for him, all else second, and he struggles to keep from thinking people from Earth are better than those from her colonies. Oh, and he has relationship problems. His wife dumped him and took off to Ganymede to do research. He still isn't over her. He's an older character and, thus, a deeper and more complex one.
Warren is a foil to Suza - age and experience as opposed to youth and talent. He's also somebody who understands the military and has entirely too much experience with war. He doesn't want to return to being the ruthless man he was back then.
Warren is often accompanied by Leroy Martin, a young American linguist who represents, as much as possible, the ordinary person - somewhat naive, even a little ignorant, and perhaps too inclined to lean on religion. The two of them are meant to be aspects of humanity. How well do they work? I'll let you be the judge of that.