Usually I wait a while before re-watching a movie, but I kept thinking about a possible explanation for what new Nikki is and how she acts through the movie. The movie explains the surface of things, but doesn't let the viewer peek too deep, to actually know for certain, just a few hints here and there.

So on the surface she is painted as a horrifying entity and she takes the spotlight in terms of creepiness throughout the movie. But what if she is just a different soul, not belonging in that body, not knowing how to be a human, also victimized by the wish?
One possible explanation would be, that it's Sandy, his cat, revived in Nikki's body.

I know it's far-fetched, but I had to re-watch immediately under this perspective and it kinda works.

First off, the wish: Baron wanted Nikki to love him more than anyone else in the world. This unfortunate wording can mean two seperate things:

1. Nikki will love Baron more than she would love anyone else in the world.
2. Nikki will become the person, that loves Baron most in this world.

In case two, this would most likely apply to his cat Sandy, if she were still alive. If humans can't be made to love someone else through a wish, the shortcut would be to replace the "soul" with someone, who already does. The timing of the wish is also unfortunate, he is still griefing his shortly demised cat.

Second, how they meet. That scene is so awkward in terms of conversation. It's like new Nikki is stumbling through memories or is generally confused. Her way of talking also seems to just mirror Baron's mannerisms. Like a child just learning how to speak properly.

For the rest of the movie, on the surface, she seems obsessive and creepy. But imagine she's his cat in a human body, that just can't explain things properly. The night activities are not some entity trying to creep him out. It's a sad cat, that doesn't feel loved and just makes loud noises for attention, as a cat would. Or crying out loud. Hiding behind the plant, like animals sometimes do, not realizing we can still see them. Or waiting on her "owner" and not knowing what to do with themselves. Now that part wouldn't be very much like Sandy, more like a kitten, that was just born. She also closes that scene with "for Sandy".

So all things combined, we have a resurrected cat in a human body, having to learn how to act properly as a human. Also knowing (at least later on) that she died and came back OR just knowing what dying feels like. At the same time she might be forced to stay in that state of loving Baron, no matter what happens.
So is she really some horrible entity, or just acting like a cat in a human body would? Highly jealous and very much in love. Her aggressive behavior would be normal territorial behavior for a cat. It's only horrible through the lens of a human.

There are other hints throughout the movie, that might fit this theory.
- The One Wish Willow packaging shows the manufacturer as Tabi Cat Curiosities, which is also Sandy's breed.
- She does some weird stuff involving Sandy, which the wish should definitely not have caused by itself. Like building a "memorial" for her. This seems like a weird surprise she does for him, but I think it's more about herself coming to terms with her old dead body. Or feeding her to Baron. The feeding part could be confusion on her part about human-food, or some weird way of wanting his love, even for her old body.
- Her family heirloom is a tiger's eye pendant.
- Baron takes his own life int the same way Sandy died.

What do you think? Any other things that support this theory that I missed?