Like most female leads in any story, Buffy has a romantic interest. Her and Angel became a team; they depended on each other and fought for each other. This makes Buffy and Angel's relationship different than most other female leads relationships because one of them was not always saving the other. They fought together. That makes their bond extremely strong, and also still leaves Buffy to be a strong female superheroine as opposed to being a damsel in distress.
When Angel lost his soul, he set out to slowly torture Buffy. Even though she loved him very much, she was able to differentiate between Angel, her boyfriend, and Angelus, the evil vampire, and to fight the vampire version of the man she loved over and over. While she couldn't kill him at first, I think she still deserves a lot of credit for being able to fight him at all. Angel and Angelus have the same body, so to Buffy, it's essentially like she has to keep fighting the man she still loves. She is strong enough to see past that and stop him over and over, which in my mind, makes her a true superheroine.