Reese was a victim of a group of vampires that had been kidnapping and transforming people throughout New York City. As the protector of those that travel at night, Moon Knight intervened, slaying the vampire kidnappers but taking sympathy on their newest converts. He set them up to live off donated blood, including Reese, who Moon Knight hired as his secretary. They keep the same hours now, so it was an easy transition.
Reese made her debut in Moon Knight (2021) #1 by Jed MacKay and Alessandro Cappuccio.
The extent of her powers has yet to be seen, but it is easily assumed that she can do anything typical Marvel vampires can do.