Although most of romance is about the boy and the girl, there is an incredibly important role in the form of the sister or friends of the main female protagonist. Without this important person, no matter what happens with the protagonist, we are unable to see a different view or side to the whole situation. We are without the often wise voice of those who can see the situation clearly because they are outside of it. Yet frequently there is a secondary storyline going on with the friends as well as with the male protagonist.
Sometimes they are after the same man. Sometimes they just aren't getting along or there is some older rivalry between the two of them. Its hard to say what the relationship is going to be, but without it the story is paler in comparison.
Where would either of them be without that friendship? The wrong guy is usually the one who gets picked when that sage advice from a external source is missed. It doesn't only have to revolve around a guy either. It could reflect family relations, other friends, heartbreaking events. Even how the protagonist sees her friend can tell you a lot about the protagonist herself. How they talk together, what they talk about, what she thinks in the privacy of her own head that you as the reader get to share by picking up the story.
Often this friend role is also the one that moves the story along. She is the one that talks to the guy on the protagonist's behalf, or she is the one to spur her on to talk to the guy or to do something about the situation herself. Without this important role the story would often end much earlier and much more tragically than they typically would. Even Juliet had her nurse to help her seen her Romeo. Too bad she didn't have more sense to share or Romeo and Juliet might have had a much better chance of ending up as a triumphant love story instead of the devastating tragedy that it became.
Choose a character to serve your purposes well in this regard. You want a character that will see the story you want to its end, if you aren't careful these characters can be the ones to send you careening off into strange directions you didn't plan on.
