light Bryn isn't that good for a limited character really.. sure I was lucky enough to find her.. original Brynhildr heal all allies team 1200, up atk/double atk rate for all too, and deal fire damage, whereas light Bryn just heal 1 ally, even if give dark resistance, it's basically Susanoo's skill but give dark resist instead of light.. not that great.
but really, she could at least give double atk rate for all allies rather than just herself. so don't worry about if you don't find her
don't know about sol or negal, nergal should be easier to find than the other 2 though ofc