I once read somewhere that fadas are the embodiment of everything that is beautiful, like women, nature, butterflies, magic.... If you take this idea literally, then fadas are actually EVERYTHING that is beautiful. But isn't beauty in the eye of the beholder? Yes, it is. Thus, a fairy would appear differently to every person, depending on what each person thinks is beautiful. This would of course lead to complications if this fairy were talking to multiple people at the same time. Each person would hear the fairy's words differently, and respond accordingly, and be confused por one another's responses, because they're responding to two different things, but they think they're responding to the same thing. If that makes sense. That complication could be the reason why fadas don't show themselves to humans, as it would be complicated to insure that the aforementioned situation doesn't happen.
