Let’s make this its own thing instead of burying it, and call upon everyone who is like “I’m interested in your Marvel’s Sailor Moon thing, but I don’t know the canon!” Because no knowledge of the canon is required or even helpful! Just ask: do you find this villain interesting, and does it have the potential to be threatening?
#9 is a tiny piece of an Eldritch God, hacked off in a battle with a mad sorceress in the world’s mythic past, and then hacked into tinier and tinier pieces until it was a little stub that could barely think or act. It insinuated itself into the lives of a mad scientist (Professor Tomoe) and his daughter (Hotaru Tomoe) through sci-fi means that make sense in context, and makes a deal to steal and copy Hotaru’s form to be able to act and think again. Motivated by a tiny sliver of Eldritch God, #9 interprets the world using the hardware of Hotaru Tomoe’s brain. (Hotaru is still there, #9 is a copy).
#9 and Professor Tomoe have a plan to summon the Eldritch God that #9 used to be a component of. This is bad for the world. #9 expresses malice and hatred for the world, but also even more intense self-hatred and disgust. She has forgotten almost everything (but what she recalls makes the Professor a warlock-engineer) and can do almost nothing (but what she still can do makes her a fearsome sorcerer) expresses hateful revulsion over the fact she is so diminished, her power is wasted and gone, she should be able to crush everyone and everything but she can do nothing. She only takes joy in the suffering of others but it’s fleeting and hollow. She regenerates from any gruesome wound and, depending on the MPAA, inflicts gruesome wounds on herself for no reason. She speaks with incredible cruelty to everyone, especially Hotaru, about their worthlessness and the futility of their lives and how they must be idiots to not kill themselves. Nobody in the mad scientist’s team really tries to stop her. Her plan to call the Eldritch God and destroy everything is a very spiteful way of committing suicide, as being reabsorbed will cause ego death and nothing else will cause the cessation of her experience.
This is not the order it is presented in, of course. You first see her just being spiteful and cruel and looking like Hotaru, being cruel to her with nobody stopping, then without Hotaru she expresses more disgust and self-hatred, etc.
A: Is this a setup for a compelling villain on its own with no symbolism?
B: Is this a setup for a compelling villain when I make clear to you “#9 is the physical manifestation of Hotaru’s mental illness in a form that can visibly assail and be confronted by her”?
Short version: it’s innately pretty interesting (with or without explicit unpacking of the symbolism), but this strikes me more as a TV-season villain or a novel villain than as a movie villain. The motivations are complicated and counterintuitive, and making them compelling is going to entail getting the viewer over the initial hump of “this person is just awful and really unpleasant and maybe she’s a fun-to-hate monster but I sure don’t care about her internality.” That probably takes time and breathing room that you don’t get in a feature-length film. I can theoretically imagine it being done with immensely skillful execution, but it’s rarely a great plan to bank on that.