Nataku is a creature named after a Chinese God-Warrior. Hir wears Chinese clothing, and has a lotus flower on hir forehead. I refer to hir as, well, 'hir', because it's a nice gender neutral personal pronoun ^_^;
Very little about Nataku was explained in the Movie. Again, hir seemed to be another plot device, solely there for battling Aoki Seiichirou. Hir met hir end with Aoki.
The manga travels much more in-depth about the beautiful genderless creature known as Nataku. Hir was originally a young girl, who was destined to be a Chi no Ryu, but she died and her grandfather attempted to resurrect her in the form of Nataku. However, he only partially succeeded. Nataku is emotionless and genderless, and is the perfect killing machine, as it is the only thing hir knows how to do.
X Japan song: