Father Anthony Forthill is a priest at Saint Mary of the Angels church in Chicago. He is a close friend of Michael Carpenter and often helps look after Michael's children when his duties as a Knight call him away from his family. He makes Harry's acquaintance before the events of Grave Peril and grants the wizard's memorable request to bless an entire barrel into Holy Water.
In Death Masks, we learn that he is a central figure in the Knights of the Cross's ongoing war with the Order of the Blackened Denarius. Under his care are the coins the Knights have managed to take back.