Normally I figure the whole childhood-promise trope is a little dumb. But here I don't mind it, because "a little dumb" is basically the cloth from which this manga is woven.
I legit thought there was going to be at least some "will they, won't they" with these two, especially playing around Kana's supposed innocence, but no, it's full speed ahead motherfuckers and I'm all here for it.