I'd assume some kind of I'm sorry. Even though Hye-Na said it wasn't out of guilt or regret, based on all his actions before he lost consciousness and the ending to the final chapter I'm guessing he has resolved to move forward with life.
His actions of visiting his brother in prison and his sister's grave, including what he says during those moments, indicate that he knows he can't absolve himself of what happened, instead deciding to live for his sister.
It's similar to a repentant murderer. You can never make up for the murder, only do good actions on behalf of the wrongdoing, living with it for the rest of your life.