She is a Xian in Chinese legend, one who never dies (at least before she was defeated by Ritsuka in the Chinese Lostbelt). Incarnated from the essences, she was someone unsure of her purpose and identity, and she felt alone, and therefore lonely and was hostile towards humans. The only being who understood her was Xiang Yu, due to the similar nature of their identity.
Because of Yu's wrath towards humanity, she could sometimes be seen a bit grumpy.