My favorite female singer from Japan is Yōko Kanno sensei (jap. 菅野 よう子). She is the lead singer of the Jazz band, called "The Seatbelts". She also composed a lot of opening and ending songs for countless Anime series as well as complete Soundtracks (for example "Wolf's Rain, Ghost in the Shell, Darker than Black, Macross Frontier", ...). She is a very talented musician. I grealty admire her handiwork.
I got to know her music when I first watched the Anime series "Vision of Escaflowne" and "Cowboy Bebop" in 1998. Since that time I simply love her music. She is indeed my favorite female singer/composer of Japan.