JENOVA is an animal (an alien who acts like one). JENOVA is an emotionless, totally instinctive creature with the intelligence and powers to understand other lifeforms but its only meaning of life is splitting, absorbing and reuniting and that is all explained in the story and the Ultimania. And it can't do and will never do something else but use its powers to fool other lifeforms, like animals who disguise as well do so it shapeshifts, talks and all that to get what it wants. JENOVA is an Id (that word is not used but fits perfectly because of the definition).
So Jenova Sephiroth is Brainiac Luthor basically.

Jenova typically makes me think of species. It even follows that in order for the species to become evolved, it had to be incubated as a hybrid.

Anyway, I've always been more of a Genesis fan. I don't really know who Gackt is. I don't like his typical style that I've seen in pics and I've only ever heard his one song on DoC. I like Genesis as he has always had more of an expressive personality.