A winning childhood friend is unheard of, but it seems like that is what Flay wants so...yay! Although, this series seems to be being setup to be a harem for the MC where he has all of the daughters. Heck, it could be that he will be set up as the next demon king, especially if he beats/kills the hero.
As for Rose, it all makes sense now. What doesn't make sense is in an earlier chapter she mumbled that she knew he would make a great teacher, but we just found out she doesn't have any of her memories so she has no idea who she actually is or about the boy in her dreams...how does she say that as if she remembered saying that to Flay when they were younger?