You should try to JASMINE, who has as a similar vibe to Hikki's first albums, and she also tried many styles. I suggest you to listen to her debut album GOLD and to her best of album.

https://www.youtube.com/watch?v=3PVS-QXuVZw

Also, Crystal Kay is another good option. She used to be big last decade in Japan, she used to sing 90s R&B, but now she moved and her last album VIVID is a great blend of R&B, dance pop and ballads. She has a similar vibe to Janet Jackson and is also an Utada fan.

https://www.youtube.com/watch?v=99YQqebbLYU

Other interesting female Jpop artists are Sheena Ringo (experimental pop-rock and jazz), Miliyah Kato (R&B, pop), Mika Nakashima (pop, ballads, jazz, rock), Miho Fukuhara (soul), etc.