This thread really is getting pointless.
All I see so far is Charismatic asking for advice on how to carry on a relationship that is clearly one-way at the level expected, and then turning around and deciding not to take advice (good advice mostly) when it doesn't help him get forward with a 15 year old.
Charismatic, if the girl doesn't want a relationship now, don't force it on her. You're 18--she's 15. You're much more mature than she is emotionally; that's what I gather with you wanting to take the relationship seriously. Trust me, unless she has the same feelings for you, it won't happen. Seriously, just stick with having a good friend. Good friends can last a long time; relationships with major maturity gaps don't. I've got about three or four good friends who are girls two or three years younger than me (I be 17), and I've left it at that. Two of them are practically sisters in terms of how close we are, and we've never taken it to the point of serious relationship.
On a more sympathetic side, I understand your position. You really care for the girl, but she doesn't see it the same way back. She just wants to be friends, but you want more than that. So take some advice from someone who's gone through this--just be friends. You can't stop her from dating other guys or messing up, but be there for her if she does. I've got a list almost two Word pages length of assholes that I need to "reason" with over how they've treated some of my friends.
Be her friend, or even better, an older brother to her. You're the mature one with experience--help her through high school.