Oh I remember this but as far as I read the "king"(Tenyuu) doesn't get "NTRd", he doesn't even show any romantic feelings towards Mairu.
Make no mistake Mairu is the only MC and she ends up being indecisive as fuck when comes to being in love but I don't know how it ends, I'll wait till it ends to pass any judgement and to start reading again because I also don't remember exactly where I stopped reading...