Best friends forever. Used to be simple when romances were a one-size-fits all category. The BFF was a device to get essential info into the story. You’ve read books like this. Ttwo friends talking until the reader knows something that will be important at some point, at which time the BFF discretely disappears, often never to be seen or heard again.
That’s how romances used to be written. But times have changed. The genre has matured. Writing is better. Characters have depth. BFFs must have depth too.
They now can be main characters in their own right. In fact, they often are. They have personalities, friends, problems, bad hair days, and often they become the main character in their own book. Maybe the next book that author writes.
As such, their personalities are as varied as the personalities of the main character. Sometimes more so and sometimes it’s hard to keep those darn BFFs from taking over!