I'm not sure why it says that it's completed. There's been some character growth, sure, but nothing's really been resolved. Still, it's very cute! You might be confused why Hiro is calling Demon King "he"; it's not some Frieza scenario. The Demon King was a dude; Mao, the student he's been reincarnated as, is a girl.

Same as reversea, I picked this up because I like my yaoi dudes buff XD. I was not expecting to be hit by a feels train. A sexy train, yes. Feels, no. It was somehow both wholesome and naughty at the same time.

Honestly, I was really hoping to get to read the novel version instead of the manga, but that hasn't been translated T_T!I think my favorite parts are the world-building (is that weird to say? It's probably weird to say), especially that conversation about how the gems from the tears are basically WMDs. The dynamic between seme and uke is interesting. Personally, I think I would've liked the relationship to be fleshed out more before the sex, because it feels rather dub-con otherwise, but YMMV and it wasn't really a mood-breaker or anything. I was a little surprised that the father just let the knight marry his son, but hey, it's a manga. Probably everything in this paragraph would have been more fleshed out in the novel-version, such things usually are.