User blog comment:Tsumou/Creating VOCALOID Music. Who is best to use?/@comment-3467648-20140809221216

My recommendation would recommend you IA, MEIKO V3, Mew and Anon&Kanon, in that order. All those girls has a good voice for the ballads, so they should work for paino songs.

Another option is MAIKA. Although she isn't a Japanese vb she also does great for the ballads and with her range and extra phonemes allows you a great degree of flexibility. The issue you need more tuning  for decrease the Spanish accent

If I want to add an English option, you also have AVANNA