If it is one of our current companions, my bet would be on Astarion. I mean, it makes sense, that he as a vampire would worship the goddess of darkness. Plus he is very chilled about SH confessing, that she is a priestess of Shar. I don't think, an evil godess like Shar would make sense for Wyll or Gale (who of course is a Mystra worshipper).
