Shota Yasojiro

His biggest regret is focusing on trying to be better than Rei all of his life. While his insecurities stem from the blonde's countless victories over him, Shota wishes he focused more on enjoying his life and doing what he loved. If he had done that, he would have had more time with his younger sister. Similarly, he would have been able to romance (Y/n) in a completely different manner, and he wouldn't have had to erase her memory to get her to love him.

Takuya Miyahara

This white-haired boy regrets letting other people influence his thoughts and self-esteem. His father often tells him he loves him more than anything else in the world, but it was only one person. After meeting (Y/n) and having her raise his confidence, he learned that he was more than he thought. He often thinks he could have been happier and more fulfilled if he ignored other people's thoughts and listened to those that loved him.

Haru Akahito

This brunette has little understanding of the world around him unless it interests him. He hardly has any sense of morals and has no regrets unless it involves the things he cares about. Thus, prior to when he fell in love with (Y/n), he didn't care to explore the world or dig deeper about the people he cared about. As she taught and encouraged him to do so, only then did he have regrets about trusting someone close to him and thinking the world was perfect and how he wanted it.

Rei Hirasi/Hoshino

Rei regrets suppressing his true feelings for so long to appease others that it ended up forming another personality. He thinks it's irrevocably responsible for tainting his relationship with his grandmother. The blonde also believes it caused him to attempt to fulfill his desire to be loved with meaningless sex; something he regrets giving away to someone worthless to him.

Yamato Hisaki

Yamato always ensures that he is acting in a proper and dignified manner, setting a gleaming example for others. His biggest regret isn't like many of the others and technically wasn't his fault. Still, the brunette regrets walking into his adopted sister's room when she was getting it on with a guy. He probably wouldn't have been so bashful of sexual situations if he'd never seen something like that when he was very little.

Akari Otsu

The blonde's greatest regret is getting sent away to boarding school. If he hadn't fought with so many kids about his crush on (Y/n) and strange behavior, he wouldn't have had to lose time seeing her and she probably wouldn't have forgotten his face. That being said, he does regret many of the kills he made enough if he truly believed (Y/n) was in danger from their existence.

Sora Gensai

Sora regrets not saving his mother from his father. He believes if he were smarter, he would have been able to get her out of his father's grasp. Similarly, he thought that if he stayed for dinner on the day of Emi's death, he would have been able to prevent her death. Oftentimes he thinks that if his mother and Emi were still with him, he would have been a much kinder and happier person.

Asuka Chinen

This orange-haired girl despises thinking about the past and rather resigns herself to living in the present and ignoring her childhood. However, there are two things that she regrets most. Firstly, she regrets letting her father get high and kill her younger sister. Secondarily, she regrets letting Michi Gensai molest her when she was younger without seeking revenge.

Kazuki Hirasi/Hoshino

He regrets not being able to stop Rei from becoming such a tormented person. Kazuki believes that it was unequivocally his fault for not scolding and teaching Rei to be a better and more caring person. He also believes that because of this, he has the blood of those that Rei has tortured on his hands, including Ichiro.

Ichiro Hirasi/Hoshino

He regrets fooling around in his childhood and playing around instead of acquiring talents like Rei and Kazuki. Ichiro believes that if he had been able to master a revered subject like the arts or sciences that his parents would be proud of him. He believes it is because of his inability to impress them that they disowned him.

???

Regrets? Probably letting anyone ever get away with touching or talking to (Y/n). Or maybe it would be not revealing himself to her sooner.