Manga

MC reincarnates as Allen, a poor boy in a crappy otome game that he was forced to play by his sister. He's just a mob character, not anyone important; but he couldn't help but notice that the only decent person in the bloody game was the villainess Anastasia, everyone else was blergh. And due to the game knowledge he knows that the city will get razed in a war, probably killing him and his mother (he's genuinely fond of his new mum). So Allen decides to foil the heroine's plans, and save the villainess.

Even if the premise is somewhat similar to The World of Otome Games Is Tough for Mobs, they play out really different; this is by far one of the series in the sub-genre I like the most (mostly due to the WN, as the manga lags behind quite a bit).

What if Bakarina was even more baka? That's Bertia, the MC. She's dumb as a brick but good-natured and lovable. The story is told from the genius prince Cecil's PoV; Cecil is one of the capture targets, engaged to Bertia, and he's actually smart enough to capitalise on her game knowledge (unlike Bertia herself). It's a fluffy romance full of heartwarming moments and no harem, just you screaming at Bertia and Cecil "GET A ROOM, YOU TWO!".

MC (let's call her "Emi") reincarnates in the body of the villainess Remilia, and tries her hardest to make everyone happy and avoid the bad end, creating a good world for Remilia because she genuinely likes the villainess. It fails because the heroine is also reincarnated and a shithead, oopsie. Emi goes into a tilt, and now the body is again under control of Remilia, the original villainess, who has been watching from the inside Emi trying her hardest. Remilia decides to actually work towards the world that Emi wanted to see, out of respect for Emi... and to get revenge against the ones who hurt Emi.

MC reincarnates as Rei, the heroine of an otome game. But instead of trying to romance the capture targets, she wants the villainess Claire instead. Yuri romance, often heartwarming, often slapstick comedy between the teasing Rei and the tsundere Claire, often serious; it talks a bit about taboo and prejudice but it don't expect serious preaching.

MC reincarnates as Cyril, the villainess Sophia's butler. And he genuinely wants to save the villainess whom he serves, from falling into darkness, so he tutors her and educates her (even if they're roughly the same age), so hopefully she can avoid the bad end and genuinely get the prince's love. Except that... well let's say that the villainess decided her own route.

A 52yo civil servant man reincarnates as the villainess. Odd premise and rather funny IMO.