I don't know how her voice would be classified by Western convention (as soprano, alto, etc.), but there was an
article published by Cambridge University Press that says
"One of the most persistent qualities of singing in both traditional and popular music since the time of the first recordings of Japanese performances is that of a thin, somewhat rasping yet strong tone produced by forcing a narrow, dense air stream through constricted vocal cords. This effect, and the techinque of producing it, is described in general musical discourse as jigoe (literally, 'ground' or 'basic voice'). It is particularly clear in high-pitched passages, where there is often a conspicuous avoidance of uragoe, a kind of falsetto - in effect a weaker, purer sound produced with loosened vocal cords. Jigoe technique is evident, for example, in the Edo period shamisen song genre of kouta, as well as some styles of minyou singing.
...
More recently, the extremely popular singer-composer of dance-style pop, Hamasaki Ayumi, almost invariably sings in jigoe when she moves into higher registers, as can be clearly heard in the choruses of 'Monochrome', the opening song of her 1999 album, A."
But that's about as much as I know.