i disagree, not all relationships start of with the ml falling for the mc strait away, unlike the prince who would use her for power and the business man who just fell for her for no specific reason, i see the demon king to not blush and fall in love with her every other minute, instead their relationship just start of as saviour and saver, the demon king saved her bc he felt like she reminded him of his sister.
i don't see any love in their relationship yet, its just that he feels the nostalgia when around her. i think that the more their relationship progress the more he will see her as her true self and that development is beautiful.
i reallllly want the ml to be the demon they go so well! (づ ̄ ³ ̄)づ