Michael is a werewolf who has quite simply changed teams—once a devoted member of Brannoc's pack, he's now shifted his loyalties to Douglas. Specifically, he performs a lot of unsavory tasks on Douglas's behalf, like beating up Sam and kidnapping Bridin.
Oddly enough, Michael's not very good at his job; he botches both assignments and Douglas reprimands him for the fact that he "took a perfectly respectable plan and pissed all over it" (5.19). Those are pretty harsh words, but then again, Douglas isn't exactly Mr. Smiley.
Michael came to be affiliated with Team Douglas through some odd circumstances—specifically, after being banished from the pack. "He is rogue," Bridin tells Sam. "We no longer acknowledge him as one of our own […] In our eyes, he is dead" (20.35). Michael and several werewolves didn't agree with the decision Brannoc and Bridin's mother made to have intermarriage between were species as a way of strengthening the pack. Instead, Michael led a coup against Brannoc, which resulted in the deaths of numerous innocent were-creatures.
Basically, Michael's a hotheaded jerk, a quality reflected in his poor, dangerous decision-making and his loyalty to Douglas.