I don't actually think anyone truly meets the Mary Sue criteria, but the lawyer probably really is the closest. The sister instantly falls in love with her and she gives Guy story relevant advice in the remake of chapter one.
That's not a Mary Sue my friend

A Mary Sue is a (female) character without any real flaws or drawbacks or struggles. The character doesn't arc, and all of their 'challenges' serve merely to prove their own knack for winning at everything they do. I've never really put much thought into it enough to have any real good examples, and it doesn't seem like it's that common anyways, at least the kind of material I consume, but off the top of my head, think of like the PC in a video game; like say, Shepherd in Mass Effect series or Dragonborn in Skyrim or something. Obviously, it makes perfect sense for those sorts of characters, but if the player makes all the right decisions, their PC can do anything and everything. Now just pretend that they're a third-person character in a story; they'd be boring and eye-rollingly annoying to follow. The lawyer is definitely not that, and is just a minor character serving a purpose like every other minor character.
In Nicki's case i believe the trauma likely comes from her toxic relationship with Brent that went on for years before and after we first meet the characters of this story. Plus all the shit that happened to her later as result of said toxic relationship.
While i mostly agree about Nicki being a very well written character i think we could say the same of others too, like Ashe or Mason or Risa and even Guy himself.
Think the author said that Nicki's past with her brothers (or at least one of them) was sexually abusive

So, I think it starts way before Brent, and in fact is the reason why she accepted a prick like him in the first place.
well we haven’t spent a ton of time with her but Nicky got dragged into nude modeling and almost into porn, she had some ptsd, self esteem issues, and is a bit jealous and overprotective. She’s also covered in tattoos she doesn’t like and didn’t want. I’m glad you like her enough to think she’s perfect anyway, so do I. She is not a Mary Sue though.
Yep, this.
Also, guys, I'm not saying that complex, layered, flawed characters shouldn't be the ultimate goal of every story, but we have to remember this is a porn VN at the end of the day... the LIs have to lean into a relationship fantasy; I mean, they don't
have to, but it really goes against the business model if they don't. And anyways, I think the LIs do have flaws, especially Nicki who I view as one of the most flawed of the bunch.
Plus, consider this: if you want to develop your own head canon of the LI's flaws, not that it would count, but there's really enough uncertainty with the LIs to do that. For example, on Ashe's first scene, she tells Guy she doesn't usually do this sort of thing with men; well, she could be telling the truth.. or she could be telling Guy what he wants to hear for fear of losing her chance with him. I kind of like to pretend it's the latter, because I'm a sucker for complications. And Nicki; she tells you that she wasn't raped at the porno shoot, but was she being honest? Or is she choosing to keep that trauma to herself? Maybe she wants to tell her best friend, but every time she tries, it just won't come out? That's not uncommon for victims.
So, really, there's lots of things that Neon gives us to kind of make up for ourselves, if we really, really want to. I don't actually think any of this is really the case, but I like to pretend there's things they aren't telling us or being completely honest about.