Blood: The Last Vampire.

In Japan, the vampire-hunter Saya, who is a powerful original, is sent by her liaison with the government, David, posed as a teenage student to the Yokota High School on the eve of Halloween to hunt down vampires. Saya asks David to give a new katana to her. Soon she saves the school nurse Makiho Amano from two vampires disguised of classmates and Makiho witnesses her fight against the powerful demon.