As far as I know, she's gullible, they're something like sex friends, he goes out of his way to sabotage people who might like her/vice versa, and they come to like one another over the 5 years they've known each other, from high school to college.
So, the MC has always been bullied. Before entering high school, he learns not to be bullied by his sister-in-law. But upon the discovery of MC yankee girl who's also very gullible, he pretends to be bullied out of curiousity. He told her a lot of things which she believes like a second-hand pussy will make her more popular among guys leading MC to "lend" his penis to take away her virginity or the first chapter wearing a school swimsuit. MC obviously loves her tsundere personality and does everything behind her back to scare away any guy approaching her. The story is their last stretch of journey together of many years of relationship.