Complex sentence: A sixteen year old high school student who was once a human before she was possessed by a youkai at a very young age, because of that she became a Hanyou, a half youkai and half human.
Split into simpler sentences: A sixteen year old high school student who was once a human before she was possessed by a youkai at a very young age. Because of that event she became a Hanyou, half youkai and half human.