Hi! I just read what you posted and I hate to admit that it was a little confusing. But that might just be because it's picking up in the middle of the story because it was much more clear by the time I got to the end. So I gather that they are in college and she's just left a frat party where her ex showed up unexpectedly? And was it his brother that came to her? Or another guy who belongs to that fraternity?
It seems interesting, but as far as your questions about characters, I'm not sure it's enough of a snippet for us to get a real sense of who they are. Nothing seemed unbelievable, and I think any girl could relate to her. I would imagine we have all run from an ex (or at least wanted to) at some point or another. The guy was a little harder to understand. I couldn't really see why he'd followed her if they don't even really know each other, if he's thinking she should make nice with her ex. If he's a guy and he came to find her, wouldn't it be because he liked her, and therefore not want to talk her into going back to see the other guy? Again, that could be because I haven't read the story. I hope I'm being helpful. (Probably not, sorry. I've never been very good at giving feedback.)
I guess I would say, what you really need is a beta reader. Someone who can read the whole thing to give you the answers you're looking for. Good luck! It sounds like a fun story.