She clearly believes she's helping Remu return to her original personality.
As for her trying to control Remu, yes, but she's not self conceiving of it as controlling Remu. She hasn't been depicted as thinking, "I know the real Remu is different than in the story. How can I blackmail and manipulate her to pretend she's like that so I can enjoy myself more?"
Controlling behavior in real life is usual not the result of some psychopath concocting elaborate schemes. It's more often because they think there is something they need to be okay that's being denied to them. Because of that, they feel scared or angry and try to get whatever that is. When someone stops them, they see that other person as the aggressor, even if to everyone else they look like the real aggressor.
Ah - I see where the confusion might be coming from.
So let's start of by saying that nobody this far believes that Niji is a psychopath, or anything close to that. I believe her psychology has been presented more closely to that of a stalker thus far - with a lot of her behavior and thinking matching up very closely to well known cases.
One fundamental thing here would be her elaborate fantasies, and how they begin to supersede reality. This is something we can commonly see with stalkers - where they have these highly detailed mental worlds about their victim. In these fantasies the targets of their affection are seen in a highly idealized light (perfectly matching the stalkers own taste), and of course as holding deep affection for the stalker.
Now having these fantasies is in itself not harmful, and rather common. To some degree a lot of people might work through their social or parasocial relationships by indulging in some sort of fantasy here and there. For a stalker however the fantasy becomes more and more consuming, until it starts replacing actual reality. This is a process that we see Niji going through - working out her crush towards Remu through fiction, but this fiction becoming more and more important and real to her. She understands Remu far better than anybody else. This fictional version of Remu is the true Remu. And so on.
Now - with a stalker - if they find out that their fictional account of their victim does not match up with reality, they will choose this fiction over reality, making up some sort of excuse why this idolized person does not behave like they are supposed to. For Niji this would be the "meddler" who is corrupting Nemu, pulling her away from her 'true' self - harming Nemu in the process.
By framing things like this - so they align with their fantasies - the stalker can then justify to themselves all kinds of morally reprehensible actions - violence, kidnapping, murder. Now these are all pretty dark, and I don't think this manga will push things this far, but we do see Niji stoop to blackmail and emotional violence - and she will probably frame this as her doing something good, as her protecting the 'true' Nemu, but that is of course not what actually is going on.
Niji is not really interested in protecting Nemu. She wants - needs - to protect her fantasy, and the fantasy version of Nemu - both of which are of course only extensions of herself and her own desires, so in the end she is only interested in protecting herself. We see this through her actions and words. Instead of trying to focus on Nemu, its all about herself - how she feels - how she has been hurt.
I don't mean to say that she does any of this intentionally or knowingly. It is all purely subconscious - but it still means that her underlying intentions are not based on some sort of misguided virtue, but purely on a selfish desire.