It depends what you mean by 'back-up' singers...in lives and such, Aoi, Reita and Kai have been known to lend their voices every now and then. I think Ruki layers his voice on the actual singles instead of using backing singers, since his voice is so distinctive...it would mess it all up if some random was singing in the background :P But obviously, on some songs like Leech and Filth In The Beauty a female voice has been used. I have no idea who they are. I don't actually own a CD or album of the GazettE's, but if you find one/buy one, look at the credits and there will usually be a credit for the backing singers if they have featured on any of the tracks. I'm sorry but that's all the help I can give you =(
