KyoZaNa✿'s Manga

For some reason, she isekai into a novel as Cordelia, a side-character who has both name & looks same as the Saintess. Thus she'll get mistakenly kidnapped by the villain, Duke Valencia then get killed later after he finds out the truth.
To avoid that pitiful fate, the 8-years old girls runs away from the orphanage and by chance meets up the collapsed villain and somehow ends up saving his life.
The villain not only wants to adopt her but also wanna raise her to be the Saintess!? Oops..
악당의-소중한-따님

When six-year-old Inés laid eyes on the handsome heir to House Escalante, she promptly made the boy her fiancé. Since noble men are all the same, she figured she might as well choose a pretty one. But Lord Cárcel isn’t ready for this sort of commitment just yet, and he spends the next decade and a half avoiding the marriage at all costs!
Luckily, that’s no trouble for Inés, as this marriage failing is exactly what she wants. In fact, he has her blessing to sow his wild oats as long as he stays out of her business.
Unfortunately, being a playboy isn’t as fun when your fiancée gives you permission. And doesn’t this mean she’s cheating on him too? Now, Cárcel is determined to change Inés’ mind about him and prove he can be the husband she’s always wanted.