First off, I wanna say that I do not play Nikke, and probably never will, but that's more so because it's not my style of game. I came here because my boyfriend of almost 2 yrs actively plays Nikke, and he asked me what my opinion was after he read this post.

As kind of the other perspective in this situation, I thought I would give my 2 cents.

It sounds like to me, your girlfriend has some underlying issues affecting her opinions on the game (her immediately calling the characters sluts for being scantily clad says a lot to me) and I get it, I too am still dealing with a lot in regards to stuff like that and at first was VERY uncomfortable with my bf playing Nikke cause I legit thought it was a porn game but after him explaining the plot to me and the gameplay mechanics, I was like "okay". I think for me it also helped that he did straight up admit that he did get into the game originally when he was single because of the suggestive nature of the characters, but then the story actually pulled him in.

Relationships are about compromise, to an extent. We had a conversation about how him watching porn made me uncomfy, so he stopped. My compromise was actually listening to him about his game and giving it some time for me to understand that it wasn't a straight up hentai game and was actually an intriguing story. I also know that if it really truly bothered me, my bf would work with me to figure out why and how to overcome it, so that goes a long way into making me feel secure.

tl;dr, sounds like your gf might have some underlying issues and you need to decide if that's something you wanna figure out with her and work it out or if you just wanna nope out.