Father

It seems the story was the Dear Hunter met a doppleganger of himself on the battlefield of the war he's involved in. Then, the Boy dies during a battle. After finding the boy's parents, who worried their son died in the war The Dear Hunter was involved in, they take him in. At first, he is hesitant and worried they will notice he isn't their son. The Dear Hunter's mom was the ex-prostitute named Ms. Terri and his father was the vile narrator of He Said He Had A Story, where his father brags to his fellow soldiers, including the Dear Hunter, about the Dear Hunter's conception. It's murky about what happened after this, as Casey wasn't clear, but it seems the Dear Hunter killed him while on the battlefield. So he is mistaken for these people's child, but having a parent love him is something he hasn't had since his mother died, so he accompanies them on a ship heading for whatever is to come in Act IV. This is taken from an interview with Casey about Act Three and a bit of it is seen in the video for What It Means To Be Alone