Yakumo Fujii

The primary male character, he became Pai's Wu and companion when a rampaging Takuhi, Pai's demon friend, fatally wounded him. Before he could die, Pai merged his soul with her own, transforming him into a Wu, which in turn made him immortal as long as Pai lives. His father had previously found Pai and promised to help her become human, so Yakumo tries to keep that promise.

Source: Wikipedia