Familiar Faces

Familiar Faces is an unmarked quest in Fallout 4.

Detailed walkthrough
Talk to Daisy in Goodneighbor. If the Sole Survivor goes deep enough into the conversation, she will tell them that she was alive before the Great War. She will also tell them about another ghoul in Hotel Rexford, who states that he was also around before the Great War.

If the player character heads to Hotel Rexford and goes up the stairs, they will find the Vault-Tec rep, who is now a ghoul. When the player character talks to him, he will immediately recognize them. It turns out that because his name was not on the list to get into Vault 111, he had to suffer through radiation, turning him into a ghoul.

If the player character keeps on talking, he will tell them that he is lonely and wishes he had a friend. They can invite him to Sanctuary Hills, to which he will respond by saying "Yes. But promise you will visit!" If the player character promises him they will visit, he cheers up and heads to Sanctuary.

Once the Sole Survivor travels back to Sanctuary Hills and speaks to the Vault-Tec rep, he will exclaim that they did come to visit him and will end the dialogue. From this point on he can be assigned to any unlocked settlement, and, if there is an open level 4 trading emporium, he can then be assigned to it.