Mika Nakashima. She's got a lot of soul in her music. I recommend her MUSIC album.
http://www.music98.net/htm/6118.htmChoose a random track. I recommend the songs FAKE, Yuki No Hana (雪の華), and 朧月夜~祈り.
Also, you may have heard of BoA if you're into Japanese music. Check out her album Best Of Soul. It has a lot of her tracks from her whole Japanese music singing career. really popular in asia. shes popular in korea, japan, and soon-to-be China.
Coming out of Def Jam Japan is hip-hop artist A.I. Great r&b artist and sometimes raps.
Check out her recent album Mic-A-Holic.
http://www.music98.net/htm/6804.htmRecommended tracks: California, Queen (great hip-hop beat), and Party.
I also recommend Crystal Kay. She's part-african american and part-korean but sings fluent japanese. I recommend her best of album CK5.
Ayumi Hamasaki got a lot of upbeat songs also. Sometimes catchy techno-ish music.
I'm not fond of jpop as much as i used to be, but this is who i recommend. i think kpop is better in my opinion. hey, explore jpop all you want.