Hardcore Japanese music...?
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Dir en grey, most definitely. Their vocalist (Kyo) is crazy. <3 In a good way, of course. (Their music videos are just as... "hardcore" as their music. T_T;)
Recommended Dir en grey (Diru) songs:
FILTH
Ain't Afraid To Die
Kasumi
Obscure
Mr. Newsman
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Plastic Tree -- Not as well-known as Dir en grey, but just as good.
Recommended PuriTuri (Plastic Tree) songs:
Yuki Hotaru
Moshi Mo Piano Ga Hiketa Nara (I love this song. XD; )
Harusaki Sentimental
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Utada Hikaru 's songs mostly fall into the "JPop" category, but a few are pretty hardcore. (Check out "Uso Mitai Na" and the full PlanitB Remix of "Hikari"/"Simple And Clean.")
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D'espairsRay ...ugh... I ran out of things to say. XD; They're really, really good.
(Check out their "Tainted World" song.)
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Alice Nine is a JRock band pretty new to the scene...
(Check out "Yami Ni Chiru Sakura" and "Fantasy.")
Here are some really good songs from other JRock artists/bands:
(First is the band and then are the songs.)
Penicillin -- Chaos
Malice Mizer -- Syunikiss, Beast Of Blood
hide -- Pink Spider
Cocco -- Countdown
UVERworld -- Just Melody, Ai Ta Kokoro
Asian Kung-Fu Generation -- Haruka Kanata
No Regret Life -- Nakushita Kotoba
Psycho le Cemu -- Love Is Dead
Do As Infinity -- Raven, Shinjitsu No Uta, Fukai Mori
Siam Shade -- 1/3 No Junjou Na Kanjou
Fra-Foa -- Light Of Sorrow
Miyavi -- Jibun Kakumei
Gackt -- Oasis, Lu:Na, Secret Garden, Emu for my dear, Kimi Ga Matte Iru Kara Remix
HIGH and MIGHTY COLOR -- Pride, Ichirin No Hana
Hyde (From L'Arc En Ciel) -- Shallow Sleep, Cape Of Storms
Judy and Mary -- Sobakasu
Ayabie -- Gothic Party Speed Session
Kagrra -- Yume Izuru Chi
Retrer en Soi -- Protoplasm, Strawberry Oblaat
D - Yami Yori Kurai Dokoku No Acapella To Bara Yori Akai Jonetsu No Arai (That's the title of ONE song. @_@ Long, huh?)
I listed a few songs for each artist because I thought it'd help you out a bit. (Most JRock songs need to listened to more than once for it to be actually liked.

Remember to listen to each song THROUGHLY. Most of the best parts are in the middle or at the end.) I don't know where you can get many torrents, but if you have trouble getting any JRock mp3s, chances are that I may have some. So... feel free to ask.

You can also watch a few music videos on youtube.com.
Oh my God.
XD; I thought you wanted harcore music, but I reread over your post and you said you didn't.
I'm really, really sorry. I'm out of it today...