Most Helpful Guy
If you are single and he is single it should be considered friendly flirting for now. It's possible that it might be something serious later but just keep it cool for now. I don't know if sibling's friends should be off limits. Maybe you should have a talk with your brother about him being okay with it happening. But don't let him be the one to decide if it will happen or not, just give him a courtesy heads up that it might happen so he can deal with it mentally and not be shocked when he finds out through someone else. I think it's awesome that he offered to be your boyfriend to stop a guy. Sometimes (the other) guy just thinks you are playing hard to get and when another man walks up acting like your boyfriend then he will get the hint. So maybe sometimes it's nice to have that. I don't ever doubt a woman can't handle herself. But why should you if you got someone who wants to help you as a friend by being your boyfriend. It's like the movie "Hitch" with Will Smith. Guy wouldn't get a clue and so he stepped in even though he was a stranger too. But the other guy got the hint then and left.