Brennan was born to a poor mother who resorted to eating raw fish when she became too weak to forage for firewood, and subsequently died as a result; leaving Brennan orphaned. Brennan was taken in by an order of priests who regularly beat him in an attempt to get him to love God. He eventually accepted Christ and became a priest, and went on a mission to Africa. There, he met an African boy and converted him the same way Brennan himself had been converted by the priests.
Brennan and the boy enjoyed a forbidden lust until they were discovered. While the boy was mutilated, Brennan managed to escape and became involved with a Satanist cult. As the Satanist order enjoyed lust and other pleasures and sins without guilt, and Brennan wanted a life like this, he was indoctrined into Devil worship. He also developed a mark on the inside of his right thigh in the shape of three sixes when Satan accepted him as a follower.
Brennan also helped to arrange the rape of a jackal in order to conceive the Antichrist. When the Antichrist was born at 6 AM on 6 June, Brennan reluctantly murdered the Thorn family's newborn baby to get the Antichrist adopted by the Thorns. Years later, Brennan began to suffer nightmares in which Jesus Christ visited him and told him that he was still welcome in Heaven if he repented. Brennan ignored the dreams, and they eventually stopped.
When he then became riddled with cancer to the point that he had to take morphine twice a day, Brennan decided to try to warn the Thorns that their son Damien was the Antichrist so that Christ would forgive him for his former Satanism. He became a priest and travelled to London to warn the Thorns about Damien. He feared evil spirits so greatly that he covered his apartment's walls and windows with pages of the Bible and his apartment's door with crucifixes, in an attempt to keep the evil out. He also followed Damien's adopted father, Robert Thorn, for a long time.
The day after Damien turns five, Brennan visits Robert Thorn at the American Embassy in London to warn him about Damien. He tries to get Robert to take communion, and reveals that he saw Damien's birth and knows who his biological mother is. However, Robert mistakes what Brennan is trying to tell him for blackmail, and before Brennan can reveal Damien's unholy origins, Robert has him removed from his office and escorted from Embassy.Afterwards, Brennan begins following Robert. One day, Brennan meets Robert at a rugby match and instructs him to met him the next day in Bishop's Park at 2:30; warning him that his wife Katherine is in danger and will die if he doesn't come. The next day, Robert meets Brennan in Bishop's Park as instructed. He explains to Robert that his adoptive son Damien is the Antichrist, and that Kathy is pregnant and that Damien will kill the unborn baby, then Kathy, then Robert himself. Brennan instructs Robert as well to go to Tel Megiddo, where Bugenhagen can show him how to kill Damien.
However, Robert does not believe Brennan and demands that he never see him again, then leaves. A storm then starts, and Brennan runs to a church to seek shelter inside. However, the church doors are locked, and before Brennan can find another way, lightning strikes the church's lightning rod. The rod falls from the roof and impales Brennan down through his shoulder, killing him almost instantly.
- Keith Jennings' photographs of Brennan all showed a dark rod-shaped anomaly over Brennan, which was more solid and closer to Brennan's body the more recently before Brennan's death they were taken.
Robert strongly disliked Brennan for claiming that Damien was the Antichrist, and because he incorrectly believed at one point that Brennan was trying to blackmail him.
Brennan and Bugenhagen apparently knew one another. The relationship between the two is unknown, but Bugenhagen did appear to be slightly saddened when he discovered that Brennan was dead.