There's so so many JP and KR novels(mostly the shoujo ones) where MC is a reader or player of some Otome-game and they die(Truck-kun or smth) and get reincarnated or transmigrated into the Novel/Game as the Villainess(every single fucking time dammit) or some extra. As plot goes as plot will go, mc finds herself entangled with the "Villain" or the bad-guy(also every fucking time too) and they have their nice and cozy romance filled drama bullshit. What I'm looking for are novels where the MC is anyhow within the "plot" of that game/novel(maybe reincarnated or transmigrated or even the rare ones where irl becomes the game kinda thing) but MC doesn't know the plot or even doesn't even know its a game/novel(maybe set out with multiple individuals who enter the game/novel and we, the reader learn about it through them but mc has no clue; or maybe with omniscient narrator that tells the reader that). Requisites: No Yaoi/Yuri No harem/reverse harem/polygamy Preferences: Completed or active translation(if its a translation) Gender of MC can be male/female as long as plot is good.
The Female Supporting Character Just Wants To Live A Good Life This fits your requirements. There is another novel but its translation is has slowed to a halt and I heard it ends with a harem.
Some similar things come to mind. I Raised Cinderella Preciously has a protagonist who only knows she's in some rendition of the classic Cinderella story as the stepmother, but doesn't know anything extra. This may be a problem for her soon, because certain people around her are...probably people with ulterior motives. She's smart though, so she'll hopefully get through with only her wit. Her only golden finger is inheriting the original owner's memories so she blends into her daughters lives seamlessly. Trash of the Count's family isn't about an otome game or romance novel (doesn't have romance at all, actually), the protagonist does know he transmigrated into a novel with knowledge of future events, and he is a cannon fodder villain character originally, but I'm recommending it anyway. Just cause. You seem bored with the usual suspects, and this story is unique in different ways, with an unreliable narrator protagonist who is relatively weak for an unusually long time having to use his brain to solve issues in a world on the brink of war, numerous twists that you don't see coming, but may have been hinted at as soon as chapter 1, and a diverse cast of side characters that really matter (even the old butler. I love Ron so much.) It's currently at the point where the protagonist, Cale, no longer has knowledge of future events. Endo and Kobayashi's Live Commentary on the Villainess should also count, I think. There's technically no transmigration, just players being able to communicate with the male lead so he can see through the villainess' tsundere characteristics and understand her better. There's also Jishou Akuyaku Reijou na Konyakusha No Kansatsu Kiroku, where the narrator is, again, the crown prince ML, who is just observing his transmigrated villainess fiance. It feels kinda typical even with the twist though. I guess Everyone Except Me is Reborn kind of counts? It's not a transmigration, but a second-chance time traveling story. It's about everyone except the ill-fated female lead being reborn and knowing the future, trying their best to change things for her sake. The last I read it, it's still only her mother who has memories of the previous timeline, but the ignorant doormat father seems to be about to wake up soon, too. None of these are exactly what you asked for, but I thought they'd stay your boredom a bit, maybe.
most game/ novel insertions have the knowledge of the truth as part of the MC's "cheat" so I don't think I've ever read one where they enter and don't they're know online/in a book. without that knowledge it seems like it would be closer to Isekei than a typical game/novel insert and I avoid those due to over-saturation. the only paid novel series I can think of is The Emerilia series by Micheal Chatfield and the MC finds out what's happening within the first novel (The Trapped Mind Project). a couple in game/ novel stories where the MC have their knowledge and cheat, but with very little romance are Trash of the Count's Family The World After the Fall Omniscient Reader's Viewpoint The Legendary Mechanic not exactly what you asked for but if your tired of shoujo, its a nice change.
I have a couple of recommendations for you. A Villainess? No, A Heinous Young Lady. MC is the villainess of an Otome Game. Not someone reincarnated into the life of an Otome Game's Villainess, but the actual person who became the character of the villainess of the Otome Game. Before the game starts, however, a Knight ignorantly into her ducal gardens shortly after running away from his charge due to being recently reincarnated. This causes a meeting between the two of them, and the potential villainess decides to chase after this knight to make him her suitor. This throws the plot of the Otome Game out the window, to the chagrin of the girl who reincarnated into the body of the heroine. It also has a manga of it being translated. Fluvia Delarose Was An Otome Game's Villain. English Original, incomplete, and MC has yet to reach the school where the Otome Game takes place. MC reincarnates into the sickly body of a secondary character who takes the execution scene at the end of the Otome Game instead of the Villainess due to the fact that she practiced a forbidden method of utilising magic after losing most of hers. MC knows this ONLY due to her daughter being obsessed with this game, and that character in particular, and thus does not know the plot of the game at all. Not that it would actually matter, as the MC's actions in the story has caused the world to divert significantly from the one that the game presented, so the game's plot would be unimportant. And the MC isn't even 12 in the story yet.