I want to reject this girl who wants more than just a hookup. Is that cruel of me?
Depends, why are you doing it?
It's not fun for me to keep hooking up with her.
I understand why this person might've wanted more from the hookup.  It's a totally normal thing to want.  That being said, you're not interested so that's the end of that.
I'm a flirt and like to keep things casual. I think it would be appropriate to reject someone like her.
I think you should reject her, just be honest as to why you're doing it.