I don't have enough information to say exactly who Tara Reade is in these stories. She could be an ordinary person with an interesting story, or she might be a public figure in a specific community.
One common theme is the struggle between good and evil. Since Tara is often portrayed as a good character in the Buffyverse and vampires are typically associated with evil, the fanfictions often explore how Tara deals with the vampire elements, whether it's fighting them or trying to understand them.