Mordecai (Tim De Zarn) is an employee of The Facility that runs the Cabin in the Woods. It is his role to act as "The Harbinger" and warn the sacrifices away from the cabin. He does this by acting as a creepy gas station attendant and telling them he can lead them there, but "getting back, that's your concern." The sacrifices must choose to ignore him and go on to the cabin.
Later in the movie, he calls Sitterson and Hadley and tells them that the "lambs have passed to the killing floor. Cleanse them. Cleanse the world of their ignorance and sin. Bathe them in the crimson of -- am I on speakerphone?"
Mordecai is generally thought of with ridicule by Sitterson and Hadley as evidenced by when they pranked him by putting him on speakerphone during his call to laugh at his prophetic-esque ramblings.
He presumably died with the rising of the Ancient Ones when they awakened to destroy humanity.