Kumbaga sa choir, c Yui nasa alto range while Ai Otsuka, Shiina Ringo, Chara, Yuki (to name a few) ang mga suprano.. the so-called "helium voice"
IMO hindi matinis ang boses nya. Ang dating sakin, usually sa chorus parts and esp when she sings live, eh pinipilit nyang mapataas voice nya.. which isnt working at all and ends up sounding really bad.
I really like Love & Truth, esp sa verse part. Its the first time I heard her sing in a low subtle voice and im loving it.