Even though you sometimes want to wrap your hands around their neck, you find yourself still trying to get along with them because you have something in common, and you keep hoping against hope that they'll stop being a jerk.
Easily distinguished from an enemy or a complete stranger because you personally care if they get hurt or need something, even though you wish you didn't.
Jim: Why do you keep talking to him, then?
Joe: I don't know, we have similar interests.
Jim: He's an idiot!
Joe: ...Yeah, but he's my worst friend, man. Don't hate on him, he can't help being a jerk.