Mr. Parker was once a neighbor of the Sole Survivor. When the effects of the Great War hit Massachusetts, he and his wife were one of many in the panicked rush to Vault 111; they were denied entry by Vault-Tec staff and armed soldiers. Realizing they were unable to secure a place, and being threatened by a minigun wielding soldier after demanding entry, they were left helpless outside the gates to the vault. Despair eventually set in and they would swiftly find themselves in the midst of nuclear devastation. They survived but underwent ghoulification and ultimately became feral. 210 years later he still roams the Commonwealth and Boston alongside his wife.
Mr. Parker can be found as part of the Named Feral Ghouls random encounter with several other ghoulified pre-War citizens. The encounter with Mr. Parker and the other ghouls can repeat, even if the Sole Survivor has killed them previously. These ghouls will level up with the player character, and may even change from one type of ghoul to a stronger type with each reincarnation, making them potentially extremely dangerous. They will always spawn together and re-spawn sporadically, meaning that even if the Sole Survivor falls to them in battle, defeats them, or if one simply reloads a prior autosave or quicksave, they won't be in the same place.