ayu's appeal lies definitely in her gift of writing uncompromisingly honest lyrics and having a poetic way with words in my opinion.
when I first discovered her, I wasn't very fond of her voice (her earlier works) but after reading all her lyrics I was shocked how perfectly she could describe emotions and experiences; and how much insight she seems to have. I was definitely not used to that level of profoundness from a mainstream artist!!
I was a fan ever since