Check out tommy heavenly6. She's really great.
is a good jap singer
Shiina Ringo (early days releases)
the brilliant green
Nobody has mentioned Onmyouza yet. They are a fantastic Metal J-rock band with a strong voiced female singer. As for Olivia her best rock songs are from her album The Lost Lolli but you really can't view any of those here on this site except for the song Sea Me. Really I'm at a loss cause you said you didn't like Tsuchiya or Aikawa...This may be a long shot but there is a duo out there called "angela" (spelled all lower case)that are mostly known for their anime songs. HOWEVER(since you mentioned not liking anime songs either) their newer songs Gravitation, Beautiful Fighters and Spiral are really something else. Gravitation is like symphonic metal( Metal with a strong melodic element in it)and Spiral is very reminiscent of Queen and Kate Bush. Beautiful Fighters is more pop rock but still unique enough to not totally qualify as mainstreamed. I have no idea if you can find them here on this site since they arn't seriously popular. If I find any more cool female J-rockers I'll let you know but until then check out Onmyouza.
nana mizuki isn't really a j-rock singer but she has one song "Trickster" that would be considered rock.
she's known for her cute high pitched voice but in this song she sings with her real voice~^^ and it's really good. this song is what really launched her career as a singer
stereopony and scandal are my favorites
To me, the best is Abe Mao!
They're "heavy metal", not "rock", but this song is on the light & technical side
My favorite is probably Jyou, the lead singer of existtrace
This is more of a J-Rock band, but they are all female...
They are called SCANDAL ~~
Anna Tsuchiya, The Brilliant Green, Maon Kurosaki
I know a good band! existtrace The only jrock female singer I listen to lol
Some of Tommy February6/ Tommy Heavenly6's songs are quite rockish...