Corporal Marcus Christensen was a soldier at the First Battle of Hoover Dam.
Marcus Christensen, along with Ranger Teresa Lutz, Private Jeffery Hui, and several other unidentified NCR rangers couldn't get out of Boulder City before it was blown up, so they held off Caesar's Legion as long as they could.
It is unknown whether or not Jeffery Hui and Marcus Christensen were killed before the explosives went off. Ranger Lutz records in her holodisk message that both Christensen and Hui should be posthumously awarded for their sacrifice and valor.
Marcus Christensen is mentioned only in Fallout: New Vegas.