It's because she has this stoic, almost friendly but not quite, attitude around her. Despite being a mass murderer. She also balances the tsun and dere sides in an appealing way, being cold to Kokuto most of the time, but never to the point of violence. She's stoic at most. Yet she still asks him over to her apartment to fuck her brains out whenever she gets the chance.
Her rampages are explained away by the psychological problems of her past and true family, much like with Shiki's killings being rooted in his family lineage, which provides an opening for a fan to forgive her for these. And at the same time, this training and great physical abilities she gained thanks to her lineage allows her to pull off some really crazy badass shit during combat, which makes her lovable.