I think that you should define that "mystical" first. You can name your appends how you like, but I think that the append should have a logic to be named like that.
When I comes to think in mystical I think in a more mysterious, wise and seductive voice. If you're a male I think in a druid voice, but if you're a girl a think in a witch female fatale voice.
So what should you do? To me you should record a deep, soft voice and, how it was said before, more in the chest than in the head or mix region. You can put effects to make your voice sound more an mystical creature if it's your wish, something like if the character has two voices speaking at the same time.
But idk if the utau will understand and don't mess with the voice when rendering. Anyway, good luck! Don't forget to tell us how it's going!
See ya~ ^^