"Into every generation a Slayer is born: one girl in all the world, a chosen one. She alone will wield the strength and skill to fight the vampires, demons, and the forces of darkness; to stop the spread of their evil and the swell of their numbers. She is the Slayer."
Some would argue that Buffy is not a superhero but I would disagree. A superhero is defined as someone with "xtraordinary or superhuman powers" and dedicated to protecting the public and has some visual characteristic (typically an outfit) that makes him/her identifiable. Buffy has "xtraordianry powers" these included superhuman senses, strength, agility, resilience and speed, and also occasionally receives prophetic dreams in her sleep, these powers all help her "protect the public" from the forces of darkness, mainly vampires and demons. As for "some Visual Characteristics", if blond hair and good fashion sense isn't enough, Buff is usually found with her weapons and equipment included her trust stakes and all popular scythe.
And like all good Superheros, she has her sidekicks, the Scoobies.