Don't do it.. You will end up creating a fake character and you'll end up mixing up the personalities because in the note you would open up etc. When in reality you can't do it. It's bad because your crush might end up confused or it may lead to another person than you.
I'm not sure if it's a good idea. It's the kind of thing that I would have considered doing or something.. Like just to see how he would react. Like I'm so pathetic that I once actually created a fake email and posed as my cousin just to talk to this guy I like. Which is probably illegal or something, and is immoral.. But I did anyway and it got me nowhere. So.. I would say no, don't do it, just from personal experience. But if you want to, hell, why not? You only live once. Girl, seriously, if you like a guy, just tell him. That's what I did and it turned out that he liked me too.. Kinda.. Then he turned into a stalker and.. Yeah.. That was weird.. But that's the not the point! No. The point is that I would just tell him instead of writing a stupid letter anonymously. Just tell him how you feel and see where it goes. Don't fear rejection. There are plenty of fish in the sea. I promise. Over half the population on the world, the whole flipping world is MEN, so, YES there is another man like him out there for you that is better than him if he doesn't want you. Sorry for the long response.