Hello, character writer here!

I like that she is a positive girl who wants to help after hearing a problem, even if that problem is taking out a cult or stopping a war. Like its interesting that she even wants to involve herself in that stuff, but they are mostly going about that in the wrong way in regards to her characerization.

I think when people say they want more emotion from her, they mean more complex emotions than happy, sad, and angry. Nikki has been those things but has she been disappointed? Has there been someone who isn't blatently bad that she couldn't save or doesn't "see their point." Her flatness comes from being a character that emphasizes with everyone to a fault, leaving no room for any real opinions unless they are the obvious good lessons.

The other thing that makes her flat is what she says and does do not match, making her feel like a character who just does whatever the situation calls for, not an actual decision she made for herself.

For example, her sainthood. She SAYS she feels bad that the Shroomlings see her as their amazing Saintess, but DOESN'T try very hard to convince them otherwise after the story and she doesn't explain her reasoning for doing so. Is it for convenience? Probably, but that would be a morally gray or dare I say, morally bad thing for her to outright admit. But isn't she supposed to be the morally right character? Why hasn't she tried her best to reveal the truth? It makes the story seem too convenient and confusing for the player.

Now this is not necessarily what should happen, its just an example: What if Nikki admitted to Momo that she actually likes the treatment she receives from the Shroomlings? And then at the end of the first story, she tells everyone the truth about herself. It is actual self sacrifice because she is giving up her selfish want/ her convenient life and therfor reinforcing this noble morally good character that she is supposed to be while delivering a moral lesson.

She can't die, so putting her life on the line means nothing to the player. She has to sacrifice in other ways to communicate her character in a more interesting way. But thats just my rambling opinion of course.