Arata, the MC's confession-thieving fake "friend" is a very basic girl-hurting playboy villain whose misdeeds go beyond what he does to the MC, and who needs to be stopped. Airi Tachibana, the beauty who accepts his confession, is mysteriously head-over-heels in love with the MC before the story even begins, and within the first 10 chapters is already dating the MC in all but name, exchanging cute messages at night, making him love-themed lunches every day, flirting while going on weekend dates, and getting cutely jealous over other girls' affection for him. It's technically not cheating, because the MC gets Arata's (sarcastic) permission to have a physical relationship with her. And there will be at least one more charming girl competing for MC's heart and attention.

So what IS the relationship between first girl Airi and villain Arata? Bully and victim, basically, and definitely not "dating." When Airi accepts his confession, the MC doesn't hear the end of her acceptance: That it's conditional on Arata completing mysterious tasks for Airi, never allowing anyone else to find out that they're "dating, " not calling her by her first name, etc. So they aren't actually dating and he never gets anywhere with her.

So what are the conditions that Arata has to meet to get the girl? So far, they involve converting him from a predatory ikemen playboy into a harmless fat, bald, friendless slob with bad skin and a busted face. It's a pretty long con and provides a half-decent reason for the MC and Airi to not be officially dating. And ruining Arata's life in a way that prevents him from seducing more girls is a perfectly noble pursuit.