Father Esteban Cortez is one of the main characters in the series.
Father Esteban was the chaplain at Ashcroft Penitentiary. It was his job to counsel and administer last rights to inmates scheduled to die by electrocution. This priest was the last person to speak to Nathaniel Arkady before his execution, and was witness to his suffering. In his journal, Esteban theorized that Arkady was either channeling spirits, or they were actually occupying his body.
On the day of Nathaniel Arkady's execution, Father Esteban was gifted by God with second sight, and able to see demons using the bodies of men like puppets. Enraged, he took up arms against these creatures like a Holy Templar, and with the others Imbued, purged Ashcroft Penitentiary of the evil within.