They must drink blood to survive. They do not age. They die when exposed to direct sunlight. They are allergic to garlic (for some reason). They mus originate from the Transylvania region of Romania. They must have an upper class feel to them, a well brought up gentleman or lady sort. They must be well-spoken. They must be well dressed. They cannot be killed except for a wooden stake through the heart, nowhere else will do. They must be able to turn into bats.
But they absolutely, positively, MUST NOT SPARKLE!!!