It could be about a boy and a girl. "We're loyal like brothers" doesn't necessarily mean that they're both boys. It's LIKE brothers, so that means that they're very loyal. But the girl betrayed him and all. Red Letter Day is a happy day. It could mean that he is happy now that he realized something and he could move on. But that's just my opinion and all of your meanings can be true! There is no right or wrong answer regarding your point of view.
It could be about a boy and a girl. "We're loyal like brothers" doesn't necessarily mean that they're both boys. It's LIKE brothers, so that means that they're very loyal. But the girl betrayed him and all. Red Letter Day is a happy day. It could mean that he is happy now that he realized something and he could move on. But that's just my opinion and all of your meanings can be true! There is no right or wrong answer regarding your point of view.