Heroines (and heroes) carry a lot of weight in fiction. Their job is to engage the reader, to make the reader care about what happens in the story. The cliche heroine--beautiful, brilliant, strong,fearless--is like a superhero, capable of anything. Superheroes never fail, and if the reader knows that the heroine can't fail, there's no reason to keep turning pages. The best heroines are flawed but appealing. The reader empathizes, and wants to follow them through their adventures. Most importantly, the reader isn't sure what the outcome of those adventures will be, and reads on in order to find out.
Antagonists are often easier to create than protagonists. They do outrageous, sometimes evil things. They surprise us, perhaps shock us. Protagonists can surprise us, too, and they should. Humans are surprising creatures, and if we can make our heroines feel truly human, we'll succeed in making the reader care about them.
Great heroines are made, not born. Their backstories shape them, as do their natures. Their family history, their relationships, their experiences--good and bad--all play a part in how their personalities and their characters develop. Perhaps a heroine has had to work harder than anyone else in her business to achieve success, and is terrified it will all slip away from her. Perhaps a hero was abandoned by his mother as a small boy, and can't trust women, though he's drawn to them. These are the characteristics that make protagonists into real people, people the reader can identify with and admire.
Conflict is at the heart of fiction. We all learn that in the early days of our writing education. Whether we're writing romance, thrillers, mysteries, or fantasies, we need to create doubt in the reader's mind about the resolution of the conflict. Compelling stories mean stories with tension, with questions in need of answers. At its heart, it's a simple formula--a sympathetic protagonist in a conflict-filled situation, and a reader who wants to know how it turns out. Complex heroines, with layers of personality, promise surprises ahead--and the reader keeps reading to find out what those are.