Although Poitevin tweaks the hierarchy slightly, the angelic structure generally follows the traditional Christian format, with nine choirs of angels: Seraphim, Cherubim, Thrones, Dominions, Virtues, Powers, Archangels, Principalities, and Guardians. (Click HERE to go to the descriptions of the nine choirs as found on Catholic Online. Click HERE to go to Poitevin's version.) In this book, the primary heavenly characters come from the Seraphim, Dominions, and Powers.
The Guardians are assigned to help point mortals in the right direction, away from sin, but since all mortals have free will, the Guardians' efforts aren't always enough to keep their charges on the straight and narrow path of morality and goodness. Mortals who are descendants of the Grigori (aka Nephilim, Naphils) don't get to have a Guardian; they must rely on their own moral compasses to guide them to the right decisions in their lives. Poitevin weaves the angelic mythology and the story of the Fall into the plot in mostly easy-to-swallow portions. In this quotation from the book, a character explains why angels have neither emotions nor free will: "When Lucifer left and took the others with him, we went to war. The agony of having to fight our loved ones nearly tore us apart and, to make it easier on us, the One took away our free will and the capacity to feel love for those near to us." (Sins of the Angels, pp. 235-236).
Here is a list of the recurring characters in the series: