THE OTHER CHARACTERS
Murphy is Harry's foil in many ways.
[A foil is a character whose differences highlight a central character. For example, in Hamlet, rash, impulsive Laertes is deliberate Hamlet's foil.]
She works within and around the system, and she only occasionally gets caught in situations where she takes it on the chin. Most of these situations involve helping Harry rescue someone. Her mouth stays firmly shut at the right times, and she's capable of the bon mot or the vicious verbal attack when needed.
She is also more of a realist about the world than Harry is.
Like Harry, she is loyal and brave, and she is willing to risk anything for the innocent.
Bob is Harry's magical computer, his inner voice of cynicism about the world and magic, and his comic relief when things are bad.
Although a supposedly emotionless being of spirit, Bob seems to develop both a conscience and a sense of loyalty to Harry as the series goes on. Several times, Bob chooses to save Harry when he could do nothing.
Warden Morgan is not only a foil to Murphy's good cop, he is a foil to Harry's helpful wizard by being so rigid in his moral outlook and his job that he no longer functions as he should as a Warden. Three Eye and the black wizard are his job, not Harry's.
Marcone has an icy predator's soul, but he's also got a code of honor as strong as Harry's. He is arguably the most interesting and unexpected secondary character in the series because he always surprises with his choices.
The moment he walks into a plot, it is hard to guess if he's there to help or try to destroy Harry, or do both. The reader and Harry must keep an eye on him and must not be lulled by moments of good because the predator will emerge again.
As I mentioned in the section on the detective novel, Sells is Harry's opposite as well as his opponent. Sells is not only the black wizard, he is the anti-Harry of the novel. He is what Harry could be if he'd made different choices.
In that sense, Sells is an excellent plot device for showing Harry's backstory by making it front story. He allows Butcher to give a wider view of magic by showing the dark side, and he is the perfect opponent for a first novel.
He also walks between the two worlds of our world and the magical world so he reflects the evil possible in both.
BIANCA, THE VAMPIRE
Unlike the other creatures here, Bianca isn't controlled by anyone else, and she's a thinking, plotting, and hating monster who now has Harry in her sights.
Her part in this novel is small, and the information she proves could be gotten easily enough elsewhere. Instead, she is a early taste of what the series will offer.