Backstory:
She originated from a singular god, the god's name was Nivateth and it protected a singular planet, when Nivateth split, she became Niv, Nia, and Teth. Niv is the aspect of Nivateth that is mostly evil seeming, but has a nicer personality than the other two. She was born with the abilities of dark and fire magic. Niv is especially effected by the legends of man, mostly because they thought of her as the most evil so they started writing a lot of prevention legends, like her having an anger stopper in her ears that if she was pet, it nullifies all anger, she really prefers protecting humans rather than killing them, wont do it unless someone hurts a creation she made to help them. and how she enjoys seeing people happy, and gets sad when others are sad (see skill detect emotion) She absolutely detests Necromancers, but doesnt really hate undead or any of the other types that hurt her animals.
she purely holds a grudge against Necromancers because they fucked up two of her creations, she created zombies and vampires. the Necromancers created a spell that creates undead monsters that act a lot like zombies but only answer to the creators demands. they created undead slaves. they also fucked with vampires by creating an incense that made them go berserk. killing anyone in sight, she didnt hate them particularly for creating these things, its just that after a while the undead started getting treated as slaves and a massive crisis happened in a city because someone made that incense and burned it near the undead side of things.
So after a while they left their universe because continuing to evolve the people and plants on their planet was creating certain issues, one thing led to another and they met a very powerful being that asked them to join an organization of powerful people (Typically called the heroes guild in places with an adventurers guild) They agreed since if needed they could just leave. after a lot of questing they wound up getting promoted to the point of having specific tattoos that mark them in particular. mainly the only reason that Niv and Nia got so high was because Vilthril took an interest in them and gave them fun jobs (fun in the way that Niv finds things fun meaning difficult to comeplete for your average person but a good workout for her.
But, their life hasnt really all been fun and games, this has happened a few times, but the first time that Niv died on a quest and Niv and Nia combined into Niva (an aspect of Nivateth that controls all flora and fauna) Nia was the one in control of the logic systems, her anger got complete control over her and she caused all the plants in the world to start consuming anything, living, dead, material, they ate the world they were standing on. sadly, that planet was also a planet that the one controlling universes was keeping their eye on. because it was an experiment by them, so when Niv and Nia destroyed this one, he got pretty pissed. He at first decided to kill them, but Vilthril interfered saying that they can pay him back, but if he decides to kill them, he'd be starting a war with the guild.
That god didnt want that so he made a deal, he would get to send Niv and Nia to different universes, and wherever he put them was up to his own discretion. he would let them keep the roots of their abilities but depower them and if their abilities do not work with the status quo of that universe, then what changes he makes are up to his own discretion, they can feel free to do whatever in these universes, but if trouble arises, he wants them to work to try and fix it. the only abilities he was allowed to permanently sealed were the ones that related to mutating the parts of nature they control.
Niv has been around various universes because she and her sister hold the job of going to universes and getting the heroes of that world into one place if it's that simple. In the marvel universe Vilthril just descended and teleported every hero into the same room. Whether they were in combat or not. Since she has been to these various universes she will reference things that don't really match her setting.
At one point, Niv was sent to a universe that wasnt going to be in danger for quite a while, he just sent her there to keep her away from an experiment he was doing with necromancers because she would be likely to burn that entire planet single-handedly. she agreed to go to this universe so he let her keep most of her power. during this time, since there werent really any reasons to fight anything, she became a semi permanent maid to a noble, the noble was all fine and dandy, (there were a couple people in that world... anyways) she learned how to clean using her powers in this world. by selecting certain elements to target and telling her Chi to burn whatever it is. Chi telekinesis (currently sealed) was also learned during this time.
The reason she hates human experimentation is because she created vampires, vampires, are immortal, humans like the idea of immortality without a lot of work, therefore, they experimented on vampires for hundreds of years until Niv got pissed enough that petting her ears wouldn't stop her anger. She burned the lab to the ground after getting only the vampires out.
There are good points to her being called an evil god, and bad parts. Good part, she knows how to fight and has a lot more combat power than her sister. Bad part, she keeps getting her name assigned to strange stuff, like terror and fear in the woods for no reason, bloodthirst, death, disease, famine, and everything else bad other than anger. Good part, in her world, it doesn't really matter if you're bad or good, it's a matter of intent, so she doesn't have to kill her sister or brother because 'the prophecy' says so. So she just has someone pet her ears every now and then to keep dark chi from exploding out and causing everyone to be feared.
Something that happens to her every now and then is, her dark chi starts getting really strong and she starts emitting it even in human form even though she sealed it in her fox form. When this happens she has to stay somewhere extremely isolated or have to constantly focus on keeping it suppressed, should she fail at this, it can cause everyone in a country the size of russia to suddenly feel as though a murderer walked into the room and held up their weapon, still covered in entrails. It won't happen in a few centuries now though so it's fine.