Disclaimer: Okay, these are all letters written to James and Lily Potter after their deaths. K? I'm not going to put names, and you guys can...guess. It is fairly obvious in most of the letters.
Dear James and Lily,
It's me. I know you most likely don't want to hear from me. I'm a horrible person and I know that. I also know you won't get this letter. Why? It's my fault. I made you pick Peter to be your secret-keeper. From now on, that name is a curse for me. Peter.
James, I'm sorry. It is my fault. It's my fault that Harry is an orphan. I can understand why Dumbledore won't let me take care of Harry. I couldn't take care of you guys when you needed me, how could I take care of baby Harry?
Lily, I know you didn't like me at first. You thought I was an idiotic, egotistical, Quidditch, playing jerk. Isn't that what you called me? Well, I am a jerk. You guys trusted me, and I feel as if I handed you over to You-Know-Who. In a sense I did. Nobody knows we changed secret-keepers. If I could go back, I would've taken the risk. Even if I died, I would've done anything to keep you and James alive. And what's worse about it Lily, is that they're giving Harry over to your sister Petunia. You know, the one you talked so highly of, that had the son the size of a pig.
I know your most likely getting jealous. Aren't you James? Don't worry, I'm talking to you again. I don't know what to do. You and Remus sort of kept me in check. Most of the time. I want to kill Peter. I'd be justified in doing so I think. But you wouldn't want him to die. I don't know why, but you and Lily don't want him to die, do you? I can't promise anything, let's just hope he doesn't walk in front of me on the street.
I hope that you guys will forgive me. If you don't, I fully understand. Just please, I beg you to forgive me, everyone else is blaming me.
You guys were the best friends I could ever have.
I WILL see you soon,