Demon brothers + Diavolo react to a MC who is the nicest person but secretly the most scariest when finally snapping at a lower demon that threatens to harm anyone they care for. How would they find out and how would they react?
And when I mean scariest, I mean MC will not hesitate to pull a 180. They are willing to kill Yandere style if they have to, but most of it would be pretty crazy and psychotic threats. (If that’s not a proper enough description just go to the SAO Abridged with Asuna from EP15 [Asuna goes insane]. Honestly that’s the best example I can think of...)
How did they find out that MC is a yandere?
• When someone tried to pour acid on Asmo's beautiful face when he got into an argument with a lower demon, MC immediately throws a knife, cutting the demon's hand in the process.
• There was one time when Mammon was accused of stealing someone's money and he was with MC the whole day and hadn't left their side. MC cut the demon's tongue with a fork and tossed it to Cerberus. Mammon didn't know about that incident if not reported by his crows.
• And when some disrespectful nobles wanted to make Diavolo give up his throne, MC had tortured them in secrecy. Barbatos was the one who found out about this, but no noble wants to admit what MC did to them as they seemed to turn into obedient slaves.
What the Brothers and Undateables think about them?
• Look, it doesn’t really matter to them if MC is a yandere. MC is not the like to just go to a rampage and kill everyone on their way. MC is not an obsessive type to begin with. They never controlled them. They never feel threatened when they’re around. For a human like MC, they feel protected and loved.
• The angels found out the truth behind MC’s behavior whenever one of them gets hurt or when someone tries to harm them. MC had lost someone because they failed to protect them. Even Luke felt sympathy towards them. Though they are worried that MC will not be accepted
•All of them are worried that the other demons will do a retribution to MC’s deeds, but MC doesn’t mind the least and said that they will accept their fate but they will not die that easily.
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I know it’s totally different from what you wanted, anon, but there’s Kidnapped!MC who totally falls into your description.