|“||God, He just, He woke up from His nap, and He just, He ripped through my hands, and He brought her back. She's my ward now.||”|
–Father Marcus on Casey Rance
He is an intense holy warrior carrying out an occupation that the church no longer acknowledges in public. 
It's possible that Marcus came from north-western England, as he briefly mentioned being born near Charnwood Forest and was sketching it in his copy of The Book of Revelation. He has also mentioned being a "boy from the Midlands."
Marcus had a rough childhood even before the church took him in; his father was highly abusive to the point of killing Marcus's mother in front of him at age seven. It's also likely that he is of at least partial ethnic Irish descent, considering that a vision of his mother spoke with and Irish accent.
Marcus is queer. His relationship with Mouse was not clear, but one demon mentions that "girls are not [his] flavour." He is shown to be interested in a man in season 1, and kisses Peter in season 2. Ben Daniels has inferred that Father Tomas "was not [Marcus'] type before, but is now."
- Since the TV series is inspired by the original Exorcist novel, Marcus' character is a rebooted version of Father Merrin.