Mr. Nelson is Randy Marsh's boss at the South Park Center for Seismic Activity who appeared in the Season Eight episode "Goobacks".
Background
After Stan refers to the Future Humans as "goobacks" in a fast food restaurant, Randy forces Stan to go to work with him. Mr. Nelson appears, saying that he is surprised to see Randy in his office. He tells Randy that he has been replaced with one of the Future Humans because they work for a lot less, causing Randy to yell out Darryl Weathers' catchphrase.
In "Bloody Mary", Randy says goodbye to an offscreen Nelson before driving The Boys home, though it is unknown whether that was him or his friend Peter Nelson.
In "About Last Night...", Randy is shown with a completely different boss, indicating that Mr. Nelson may have died, quit, or was fired between his appearance and that episode.
Appearance
Mr. Nelson has balding brown hair, a large brown beard, and brown eyebrows. He wears a teal collared shirt with a red tie, a black belt, brown pants, and black shoes as well as a wristwatch on his left arm and small reading glasses. He appears slightly overweight.