That right there is my point. The fact that you say "he was a wimp that got walked over", as if being bullied by other people and having your self-esteem trashed is somehow your fault. Obviously, it isn't and anyone that suggests as much is very clearly a toxic person. I should also point out that while he changed his physical appearance and changed his mind on getting revenge, he also very readily forgave them for what they did. If he hadn't, then he would've outright rejected Emma's proposal which she basically forces on him. She basically admits this and even makes a point of copping to the fact that she's doing the EXACT same thing that she did in high school, which defeats the entire point of his "giving them a chance to prove themselves" because Emmma already failed. Not to mention, he is meant to take on the task of convincing Aine to marry him, despite his having very little reason to help her or even improve the life of the woman who bullied him for what seems to be more than a year.
If I were writing this, I would have had the main character immediately shut Emma down with her nonsensical request since she never bothered to apologize to HIM but only did so to his mother. On top of that, one of the first things she says to them when they're in private is call him by the name she used to bully him at Aine's command. It's ridiculous to have this entire journey start with a guy acting like a complete doormat and letting his former bullies come back into his life, demanding that he save their lives after they gave him some pretty severe confidence issues. Issues that, to this very day, he would likely still be contending with! So, while he may have let go of the anger (or at least thought he did), those old emotions would still flare up and he would have absolutely zero reason to indulge Emma. It just makes more sense that way. He should have more self-respect for himself and you should too if anyone who's ever hurt you comes up to you and acts as though that everything is water under the bridge just because time has passed. No, things aren't just "water under the bridge". They hurt you and refused to apologize for it. Those people are not owed your respect nor your help and neither do they deserve it until they treat YOU like a proper human being. Refusing to help them or have them in your life doesn't make you a bad person as some are suggesting. Doing otherwise is what makes you a doormat.
This isn't about revenge or treating any of the girls like shit. This is about a human being demanding some small measure of respect from the person asking them to help them after THEY hurt him incredibly badly. Kei is being unreasonably nice to a bunch of people that neither deserve or have earned his good will, much less a place within his life as roommates or even friends. They hurt him, life went on, and now it seems like none of them cared at all about what they did to him and have the audacity to ask him for help. FFS, the felt so torn up about what happened that he literally went out of his way to change his physical appearance to make sure that NO ONE ELSE would ever call him piggy again. He didn't do it for his own peace of mind, but because of that specific experience that happened in the past. The game makes that abundantly clear. You don't just overlook that and have the guy just randomly go along with this fucked up plan of marrying his former bully who doesn't even know who he is. Honestly, at this point, I would fully expect Aine to just tell Kei to fuck off after he brings up the conversation of how she bullied him when he was younger and fatter, yet she treats him with a small amount of decency because he looks more physically attractive. In short, I'm not averse to the girls redeeming themselves, but at least make it clear that the MC actually isn't going to just let all of that shit slide as "Oh, we were just kids" nonsense that people like to throw out there to let bad people justify their own bad behavior.