He is a scammer that hangs around near the South Boston police precinct. He offers a charge card to passing travelers for 100 bottlecaps, plus a 10 cap service fee. He claims that he's been working with every shop in the Commonwealth to get rid of bottlecaps, stating the charge card is good for use at any vendor in the area, but of course no merchant will accept it as valid payment except for Brooks in Far Harbor.
Parker calls the player a "retard" whether they buy his charge card or not.
After buying his charge card, the player may pickpocket or kill him to get their money back.
On approaching Sheng Kawolski in Diamond City after having bought the charge card, he will say that Parker has "got another one", and tell the Sole Survivor that they might as well dispose of the card since no trader in the Commonwealth will accept it.
His character file seems to have been copied from Ron Staples', including the typo where his special vendor container checks to see if he has been assigned to a trading emporium. As a result if brought into a settlement using console commands or tier 3 Intimidation, he has options to be commanded or moved as a settler, and when assigned to a trading emporium will sell rare food items.
In order to use him as a settler, player first needs to open the settlement menu and move him to the settlement before assigning him as a vendor or he would walk away (like how a settler is moved from settlement A to B).
Killing Parker does not count as murder, so no companions will react negatively to doing so.