Harvey was a trader who came to Nuka-World prior to the raider invasion that led to the three raider gangs controlling the park, killing or enslaving most of the inhabitants. His family was killed, and he was placed in a shock collar amongst many other captive traders, whom he now considers his family.
When first encountered in the transit center, Harvey appears to be injured and tells people that he escaped the raider-infested but wealthy Nuka-World, and that his wife and son are still trapped there. If the player attempts to help or question Harvey too much, he will admit that he is pretending to be injured, and being forced by the raiders to lure anyone he can into the Nuka-Express which leads to The Gauntlet, an inescapable torture maze resulting in certain death.
Random encounter in the Commonwealth, the Sole Survivor can free him after some dialogue.
By passing a persuasion option, Harvey will reveal his true intentions.
By passing a hard persuasion option, he can be offered a stimpak.
He can later be found in Nuka-World, choose to spare or kill him. Harvey has no weapons and will run, and any nearby raiders (and companions) will open fire on him if the Sole Survivor chooses either of the two "dead man" options.
If you choose the conversation option "You're a dead man" when you first run into him again in Nuka-World, everyone attacks him but he never actually dies. He just falls down and gets back up...only to be instantly targeted again.
If Open Season is completed, speaking to Harvey will change his dialogue, as he shows gratitude for your rescue, and is very happy with your decision to aid the traders of the park and totally remove all of the raiders.