Given that they aren’t a true vampire, the backstory could involve them acquiring the vampire persona from some kind of fascination with the aesthetic — maybe they stumbled upon old vampire folklore, a vampire film, or some romanticized idea of what being a vampire should mean. They might have been an outcast or just someone who didn't feel like they fit into normal society, so they invented this larger-than-life persona to gain attention, admiration, or to escape from their own insecurities.