People who bum off other people are the worst. Seriously. I know this one guy that who every time he communicates with me, it's either him asking for money, food, or a ride with no intent of any form of reimbursement. It's pathetic.
My advice is to make new friends. Don't make this guy your only social connection. It's risky and unhealthy.
You could always literally smack yourself in the face and say, "I don't need this."
But you have a crush on this creepy bummy guy, no? And you're sister says no, you shouldn't hang out with this guy because he's a bum. It's nice she's offering you advice but really, it's up to you.
If you have a crush on this guy and you like him like that, then see what happens. If you don't, then move on. Do what your heart tells you. Advice is nice but ultimately, it's you who makes the decision.
Well, stop being so emotional and start being more rational. That sounds harsh but you have to examine the areas that need work in your life and do something about it. Just because he's the only a$$ that gives you attention doesn't mean you have to marry him... Life can get quite lonely at times to my understanding and that need to be with someone can be pretty tough, but hang in there, the solution will come with time. I see a bigger "problem" (I don't like using the word) with you not having a circle of friends at school, there's bound to be some people you can relate to, you just gotta find them, but from what you've said it sounds like you're an introvert and it's pretty hard to just go up to some random person and chat because of that. Try hobby groups or after school things, sports, and stuff, could help~ But with all that said, it's your decision and no one should or can force you into doing things you don't want to.
We can't help who we fall for. But did you fall for him? Or are you lonely? Sit down and think long and hard about this, if you really like him for him and really want to be with him, then find a way. If not, moving on should be easier once you realize your just lonely.