DrAnthill
Look, either you are familiar with KNT, or you're lucky. I won't waste time introducing you to the original plot. If you don't already know it, do yourself a favour and don't look.
I have, personally, not read KNT and I intend to keep it that way. Everything I've learned about this vile tome of depravity is from discussions in anti-ntr circles and several chatbots based on various points of the story.
I'm not a big fan of anti-ntr fics where the poor MC becomes some cruel strongman and enacts revenge, because that's basically agreeing that the Antagonist was right. It WAS about being the strongest, most dominant man, it's just that now that man is MC. Hiroki doesn't need to become "Bert 2.0: Now with less melanin", because even one Bert walking around is one Bert too many. Hiroki needs to learn that there's no shame in getting help from others, especially if he has already tried and things turned out to be too much for him to handle. So, in this story, Bert meets his match in somebody else than Hiroki. His foil. His antithesis. Not a "SuperBert", someone who's just like Bert but simply better at it, but an "AntiBert", if you will.
Meet Dr. Akira Mori - Hiroki's older, adoptive sister. She has recently returned home after a long stay abroad, and she's NOT letting any of this nonsense slide. Soon, Bert is going to find out very well what happens when you anger a calculating, slightly unhinged expert on the human brain who's got what can only be described as a "beta fetish". Follow Dr. Mori as she puts a stop to the bullshit happening around her, fixes several other NTR works along the way and leads the whole cast of KNT to redemption... whether they want or not.
Also, here's the ages for some of the characters since no, Hiroki is NOT thirteen no matter what the accepted headcanons say.
Hiroki, Nao, Bert - 18
Ayumu - 17 (would have made her 18 too, but she's canonically one year under Hiroki so screw it)
Kanako - 20
Akira - Late twenties
Kaede - mid forties