Marko

 is mentioned in a note titled Dead End found on a wastelander corpse directly north of MDPL-21 Power Station.

Background
Apparently, he killed the note author's family in front of his eyes and let the author live who then started following him, seeking revenge. It is possible, since the note is near The Pitt's entrance, that Marko could be Marco from The Pitt. He could have changed his name by only dropping the k and adding a c.

Appearances
 is mentioned in Fallout 3 but does not actually appear in the game.