I just thought about Ayu's voice while I was in the car today
lol.
I
love Ayu's voice. It's not amazing, and if you compare Ayu's voice with someone else voice it doesn't sound so excellent. But I noticed Ayu has one of the most important thing for a true singer. Her voice is full of emotions.
While she sing a sad song, her voice sound sad like for a happy and cute song as well. Her voice sounds way different while she's singing talkin' 2 myself than when she's singing glitter. Some singers have a powerful and amazing voice, but they're able to sing a sad song with the same voice as a happy song. You may be amazed by his/her tecnique, but it sounds emontionless.
For me a singer must spread a message (whatever message it is) with emotions. And Ayu make it almost perfectly. She need more practice in my honest opinion. If she keep practicing, her voice would be perfect.