I have a question to native english speakers: Since the "friend" word doesn't have gender in english, how common / natural would be someone saying "so you're one of the male / female friend" in a real life conversation?
In my experience, it's rather uncommon since you don't usually mention gender and just say "friend" only. In this situation, since Haruki was originally a boy, Aniki acknowledges the gender of the friend because Haruki usually hangs out with guys.
Obviously, you could say "boy friend" or "girl friend", but those can confuse people since those terms are also used for someone you are dating (boyfriend/girlfriend, with no space in between words). So in the case you need to mention gender, male/female is more descriptive.