Let's look at the show
-Asuka uses her breast for a physics demonstration and is disappointed Shinji doesn't take the bait.
-Other flirty things I can't remember.
-"Want to practice kissing?"/"He wouldn't even hold me."
-Thinks Misato is a slut, and wonders if she's going to be like Misato when she grows up.
-'If I can't have all of you I want nothing to do with you' (or something from that effect) from the third impact train car scene.
-Accuses Shinji of not wanting her specifically, but wanting anyone to ease his loneliness.
-And let's not forget, she's 13/14
Now the fandom is not necessarily the show, and fanfiction kind of takes on a life of it's own. For example Hikari is usually portrayed as a kind sweet girl who anyone can approach, but in the show she's kind of bossy and only really shows her sweet side to Toji and Asuka, and the fact that she would be friends with someone like Asuka shows she's either chasing social points or gets along with a girl who's narcissistic and mean.
Fanfiction obviously portrays situations that didn't arise in the show, and I've seen a lot of takes with various levels of believability. Asuka's age is a major factor.
Unfinished. Asuka get a job in the Geofront because school is boring. She becomes obsessed with getting a reaction out of Gendo Ikari and tries to seduce him. Disturbed me more then any other Evangelion fic because how believable and in character it is.
KAdam AU. Asuka is a college student. She seduces (date rapes) Shinji, the boyfriend of her Japaneses Tutor Hikari. Not that believable. Asuka is attracted to Shinji for no real reason. Portrays Asuka as a genuine Narcissist or Sociopath when in the show she has a glass ego that she uses to hide behind. Like many of KAdam's works it's a competent piece of writing that is not about evanglion.
Misato makes Shinji and Asuka pot Brownies and the end up playing strip poker. Disappointingly includes no sex or threesome. ehh, it could happen.
I also remember one fic that I read until Asuka gives a hand job to a homeless guy so he would buy her booze. I stopped reading there, BUT Asuka is majorly messed up and I think she has a personality disorder (histrionic)
So how promiscuous is Asuka and how does this effect her relationship with Shinji? I'm writing a fic where Asuka is a 19 year old University Prof and Shinji a student at the University she teaches at. I imagine that Asuka would have lost her virginity where she was around 15 to some college guy while she was a PhD student, and either engages in emotionless casual sex, or has a series of relationships that end when the guy can't stand her anymore, or has long relationships where she doesn't feel strongly about the guy because she doesn't feel strongly about anyone.
Shinji may not necessarily be a virgin, but I don't imagine him being very experienced. He's depressive and introverted, and I can't imagine Asuka being attracted to him, despite that they were literately made for each other. Asuka doesn't let herself be vulnerable, and Shinji doesn't know how to reach out. I don't want Asuka to be some Tsun that likes wimpy MC because he's MC.
What do you guys think? I'm probably going to write there relationship as two scabs picking at each other, but why would they get together in the first place?