Tav doesnt really have a backstory like our companions - Astarion with Cazador, Wyll with Mizora, Lae'Zel with Vlaakith, Gale with Mystra, SH i dont even know who with, to select their nemesis/god/teacher/influencer/whatever you can come up with. So the best guess with the CUSTOM character to add someone that would haunt their dreams in order to twist them to do what it wants, would be the person they find physically attractive.
Its just a simplification to avoid specificity, and we as players can come up with whatever we want. I don't see a problem in this.
If would be pretty interesting if you could choose to make "the one you dream of" doppelganger of one of the companions. Talk about mind games.
I use mods to create them on some playthroughs. Its really creepy in game.
