I'm going to give it to you straight. You're not going to like this, but you need to hear it. You've made the nieve mistake of buying into the"it's all about personality",myth when in reality it really just boils down to this.
physically he probably just isn't attracted to you much. I know it's hard to hear but honestly with guys it's like 75% looks, and if you don't pass his standards there's little much a change of personality will do you.
make sure you are in shape, loose glasses and get contacts if you have any. start dressing more sexily, start using makeup to make yourself more appealing.
This will help out to a degree with future interest from guys.
beccoming a "Bad girl" won't help you out much at all.
The personality change that would help you out with interest from guys is if you became a slutty girl.
A slutty girl will have appeal to many guys even if she's nothing really special to look at. From the way you describe your rival that is possibly what it is. You assume he wants her for a long time gf, when in reality he probably just wants to have some casual sex. .
I'm sorry for this post, but it's better you know the truth now rather than buying into this "personality" fairytail/myth which is peddled everywhere
it is "TYPE" which is the most important thing in dating.
If you are a persons "TYPE".then often you will be granted some leyway on looks. Infact "TYPE" and ":LOOKS" are closely intertwined because the type of guy/girl you go for influences how you rate someone elses looks/appeal.
If you are not a persons type, then to have the same chance as a person who is that persons type you need to be noticeably better looking.
best advice I can give you is to work on your looks.
anyone here telling you to work on your "personality" lacks any real understanding of the dating game