Why does the manga girl's boyfriend regret?

It could be due to various reasons. Maybe the ugly guy has some unique qualities that the girl values more than looks. Or there might be issues in the girl's relationship with her boyfriend that led her to make this choice.

It could be that she met someone who seemed more exciting or offered something her current boyfriend didn't. Maybe she's going through a personal crisis and makes a bad choice.

Maybe she realized later that the nerd had qualities she overlooked initially.

Maybe she was really hungry and out of control. Who knows?

She might be blushing because of a secret she's keeping or because she's caught doing something she wasn't supposed to. Maybe she's also blushing from happiness or excitement.

Maybe she's stressed and sees smoking as a way to cope. Or it could be part of her character design to make her look edgy.

Maybe she lies to protect herself or someone else.

Maybe she's sad because she lost something important to her.

Maybe she's just really tired from a long day of adventures.

Sometimes a girl cries in manga because of external pressure, like family expectations or societal norms that she can't meet. Another reason could be facing a major setback or failure in achieving her goals.