Bestfriend are a very special and strong friendship that been friends from more than a few months and have gone through hell and still made it through together. Between a girl and another girl the Bestfriendship is usually very strong if it's not that I don't think "bestfriend" is the right title for that person. A boy and a girl can be Bestfriend but if one of the two is in a relationship it is their responsibilities to make sure it's nothing more. A Bestfriendship between a boy and a girl usually ends up as a relationship, after a while they start to grow feelings.If a girl has a boyfriend and she also has a "Bestfriend" that is a guy she probably has feelings for him(not all the time though). I'd a guy has a girlfriend and he has a girl calling them "Bestfriend" she probably has feelings for him.
My Bestfriend and I have been friends since 3rd grade.