Summary:
She auditioned because she wanted to meet Okai. Back then she was young and didn't think much more about it than "If I become Morning Musume/H!PEgg, maybe I get to meet Okai-san!" And that was why she auditioned and stayed in eggs in the first place.
Eventually she started thinking more about it, and about wanting to become a professional singer and debut. She doesn't like her own singing that much though but if she could get to grow and improve, then she would stay in eggs.
Then when she got picked for Juice=Juice she got insecure. But because they were indies, she stayed and figured that she might improve that way too. However, then the major debut was decided.
Aina herself isn't (wasn't) happy with her own singing ability and despite being praised by people, while that made her happy, she didn't feel that way. She also sees idols as girls who everyone thinks is cute, but she doesn't fall into that category. Nor is she that good at singing.
She thought about it a lot and talked to friends and family, but decided that she didn't want to debut because she isn't happy with her singing and even if she would improve, she was afraid that she would end up just letting herself stay the way she is because her fans keeps praising her.
Friends and family asked her over and over again if she was sure, but she was and thus she quit. She still wants to become a singer and improve and such, but right now she's not an idol nor an artist just yet. Just a high schooler aiming to become a singer. She also doesn't know how long it'll take her to get happy with her singing, maybe 10 or 20 years or maybe she won't ever become happy with it, but she'll try to.
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I still think that she would have been a great personality in Juice=Juice, and while I don't think this is the whole truth... as long as shes' happy, good luck Aina~