Really hard to write in the point of view of a character you hate but I'll try. Here is Charlie's letter to Riley. Don't hurt me if it's terrible (I just don't like Charlie).
Dear Riley Matthews,
Strangely, you don't know just how amazing you are. You're beautiful, funny, smart, and charming. But you're constantly in the shadow in your best friend. All the guys prefer Maya, who's supposedly "prettier and mysterious," but I definitely don't agree. I like you. You deserve somebody who sees all of your beauty.
I've liked you since seventh grade. I couldn't help but watch you from afar, you're just so fascinating. But all that time, you were in love with Lucas Friar. I guess I could see why, he's handsome and Southern and whatnot, but why him? He can't even see how awesome you are, he can't appreciate it. And in the end, he chose your best friend. I just couldn't stand it anymore.
You deserve a guy who would do anything for you. You need someone who knows how amazing you are. When I saw that Lucas didn't ask you to the spring formal, I just knew that he didn't deserve you if he can't make you happy. I can, but you still like him better somehow. Is he hotter or something? I changed my hair for you, is that enough? What do you want from me?
I've waited all my life to go out with you, but when we went to the movies, it didn't feel real. We didn't even talk. Will you ever like me as much as you like Lucas? I don't even know if we're official or not. But even though you frustrated me during our date, I still like you.
You're way more beautiful than Maya in my opinion. You're adorable, charming, irresistible, yet you still want someone who can't see all of that. You don't even see it yourself. You deserve someone who can tell you you're beautiful.
Can you even see the way I feel about you? Why can't you just give me a shot? I can promise you that I'll be a way better boyfriend than whatever Lucas can offer you. I want to give you the world. All you have to do is give me a chance.
Love, Charlie
These things just get shorter and shorter. Remember to give me a few suggestions (I don't think there any more characters left).