The protagonist set his sights on Karen.
By chance, he discovered that Karen had been bringing erotic doujinshi to school. Using that knowledge—and her apparent fondness for him—he steered things toward sex.
Under the guise of being her boyfriend, he took advantage of Karen’s timid nature, doing whatever he wanted with her.
But Karen turned out to be far more lewd than he’d imagined, and she didn’t seem entirely opposed to his advances.
Originally, the plan was to play the role of a loving boyfriend and gradually mold her into submission. But after seeing just how perverted she was, he figured there was no need to take things so slowly.
Around that time, he also learned that Karen had always harbored fantasies of being molested or even raped. To expose her true nature all at once, he lured her onto a crowded train, where her desires were fulfilled.
Then, the protagonist revealed his own true intentions, telling her to become his sex slave.
“If you become my bitch, I’ll fulfill every one of your dirty desires.”
Karen, being earnest by nature, felt both drawn to and embarrassed by the idea of becoming a sex slave.
To help her shed her inhibitions, the protagonist introduced her to Machiko and Haruka, who had already embraced their roles as submissives. Through experiences like being used as a fleshlight by students or getting fucked by strangers in public, Karen gradually came to accept her own perverted nature.
Grateful to the protagonist for bringing out this side of her, Karen thanked him and confessed that she wanted to become his personal onahole.
And so, she began living a fulfilling life as his devoted submissive.

[Translated from dlsite]