The village shaman. She's having a hard time influencing Kurisu (Merrin was sent here after Kurisu was in power for a while). She's also allergic to dogs, which makes this an unfortunate assignment for her.
Merrin has not appeared nor is he mentioned in any published game. Merrin was going to appear in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.