Marcus is the vampire that is second in command under Bishop. He helps Aidan remove the body of Rebecca in Episode One, and later nearly gets into a fight with Aidan while trying to assert his authority.
While under the influence of Ray, Josh beats Marcus, overpowering him with his temporarily enhanced strength. Marcus did not take the humiliation well. Without Bishop's orders, he brutalizes Josh's sister Emily outside a club. This act, and is failed attempts to lure Aiden back, have caused him to lose Bishop's favor recently.
he severly injures Emily
Warned by Aidan, Josh moves with Emily back to the family's house, only for Marcus to follow them. After a brutal fist fight with Aidan, who demands Marcus keep their rivalry between them and keep Josh's family out of it, Marcus retreats.
Marcus is later shown to be in contact with another vampire coven. These vampires appear more traditional then Marcus' brood, sleeping upside down in burlap sacks to protect them from the sun and using humans as familiars. It is unknown if Marcus if a formally contacting them on behalf of Bishop or is their spy in Bishop's coven.