She is a celestial maiden that wants to find the celestial robe in order to be immortal. When the crew kills Naraku, she pops out and makes everything stop in time so she can have eternal night. Naraku wants her body because its really powerful and he also wants to be immortal. This all happened 50 years ago when Master Monk Miyatsou puts a spell on the young demon. The celestial robe is taken to Aki Toki Hojo and he told them if the full moon shows for more than one day burn the celestial robe in the mouth of the Mt. Fuji Crater. That is all I really know about Kaguya.
Submitted by Sango