I had some plausible deniability about the whole "we met before when we were really young so you don't remember" thing, since it's a very popular cliche as stated before, but
this page has me thinking it won't be like that. Saying "imagined" is a bit too specific of a word choice for someone to say about another that they likely would've remembered meeting before. Could be an isekai, or maybe she saw him one time in the far-off distance doing something noble and helpful (there's a difference between meeting and seeing, after all).
On the off chance this
does become an "observant isekai", It'd be interesting if this develops into a personality reversal of
Jishou Akuyaku Reijou na Konyakusha no Kansatsu Kiroku. Although, even if it doesn't, it's nice that another manga's angling itself as such at all, since that's a bit of an uncommon story trope.