For me:
1. Her big beautiful eyes

2. Her adorable voice
3. Her wonderful personality and demeanor.
4. Her individuality (as in she's different from most artists) and writes about other things besides love <---too cliche. This is one of the things that made me really like Ayu
5. How she's such a real person and stays true to herself and encourages her fans to do the same.
...Ya know, naming off just five things is hard cuz I love Ayu for countless reasons