Michael is referred to as the Fist of God by Harry Dresden on a few occasions. He is a modern day knight who has a wife and kids and an F-150. He also has a sword called Amoracchius which has a large spike set into its hilt; one of the three nails from the Crucifixion. (One character is told this and says, "What, you mean the Crucifixion?) Michael has been entrusted with this sword, by whom we don't know yet, and he smites the enemies of God with it. He is a rock solid character who would assault the Gates of Hell if God told him to do it. And he would win.
One of his best lines comes when he and Dresden are about to attend the vampire ball. Dresden tells him that there might be innocent people in there being prepped as food and Harry tells him not to do anything stupid like try to rescue them, thus breaking the truce and bringing unholy retribution. Michael just looks at him and says, "I am what I am." Dresden sulks.
As I said before, Michael is not played as a stereotype and that is refreshing. He admonishes Harry every time he swears. He warns him not to mock God. Harry generally behaves himself around him. There is also the gentle persuasion that Michael just exudes by his mere presence. The novels have established that Dresden does not agree with God's hands off policy and just doesn't understand why He can allow all this bad stuff, that Harry feels is up to him to clean up, happen. This book has the two of them working closely together for the first time although they have a somewhat close relationship from the past. Harry is seeing how the power of God works through His creation and is starting to come around. As I understand it, Michael will appear in later books as well. It will be good to see him progress and we will all be rooting for him to bring Harry Copperfield Blackstone Dresden to the Lord.