A Sacramento reporter, Mark Hoffer was writing an exposé on McNamara Engineering's California Bullet Train and its inadequate safety procedures. His confidential source was Stephanie Dobbs, with whom, for a brief time, he was also having an affair. When Chance boarded the train, he soon found a string of calls and texts between the two, and figured the jilted Mr. Hoffer was a candidate for wanting Stephanie dead. Guerrero's research, however, proved otherwise, since Mark was already several days dead, blown up by the same sort of plastic explosive someone tried to use on his ex-lover.
The affair seems to have been ended by Stephanie, but with no indication of resentment on either side.
Hoffer is never seen onscreen or in a picture, nor even directly quoted.