- Jovian and Jacqueline Kikion, hands down. Improved versions of Apoch and Astreal augmented with Alucard's blood, making their attacks virtually unblockable and their barriers virtually unbreakable. On top of that, they're absolutely Ax-Crazy Psycho Lesbians, and Psychopathic Manchildren who commit all manner of horrible deeds, such as Cold-Blooded Torture, rape, and wanton mass murder and destruction, purely "because it's fun." In-Universe, Akua and Kahlua, badass vampires in their own right, are terrified of them because of their sheer power and penchant for Unstoppable Rage and destruction.
- Nowhere is this better highlighted than their treatment of Felucia. They constantly torture her with her magic and sexually assault her for days on end, and Felucia's nature as a doll demon means she'll heal from all of it and feel every second of it. By the time of Act IV, it's happened so much and so often that Felucia is beginning to lose her sanity.
- In hindsight, it actually makes Hokuto, their master during Acts III and IV, even worse. When resummoned by Talon in Act IV, they explain to Rin that their morality is shaped by their master/summoner's, and they're not naturally good or evil on their own. While Hokuto was already a nihilistic sociopath (though he was very good at hiding it), the fact that Jovian and Jacqueline were that horrifying and evil under his control speaks volumes for just how much of an unhinged monster he really was.
- Dark is usually calm and very much The Stoic, but when pushed, he's prone to borderline Ax-Crazy fits of rage that even terrify his friends; when he gets to that point, don't expect the corpse of whoever pissed him off to such an extent to look pretty by the time he's done. A perfect example is his killing Miyabi Fujisaki in Act II, which also doubles as an awesome moment. While Tsukune and the others have No Sympathy for Miyabi and agree he deserved every second of Dark's wrath, especially since he raped Mizore and bragged about it to their faces, they're still horrified at exactly what Dark does to Miyabi as payback; in this order, Dark slowly crushes every bone in Miyabi's arms and legs to powder before ripping them clean off one by one, punches him in the face until his lower jaw snaps right off, and finally rips his spinal cord out. All of this was with his bare hands, and Miyabi was still alive and conscious until the very end; Yukari understandably faints on the spot.
- Arial Kuyumaya, as revealed in Act V and expanded on in Act VI. Dark's guardian angel, the one who freed him and set him on his current path... is a near-psychotic Bratty Half-Pint with a Hair-Trigger Temper who's madly in love with him, taking it to Yandere levels. Gabriel remarked that every time she even suspected that Dark had a girlfriend, she would lose her temper and destroy her home in Heaven, and when she discovers his relationship with Mizore, as soon as she gets the chance, she uses her ice claws to literally tear Mizore apart in a jealous rage right in front of everybody, and is only stopped from finishing her off when Razico knocks her out; by the end, Mizore is so horrifically injured that Ceal has to transfer organic matter from deceased HDA officers to actually heal her. Later, in Act VI chapter 13, she goes into Mizore's room in the middle of the night to steal her engagement ring and outline to Mizore exactly what she'll do to her if she doesn't stay away from Dark; Troubling Unchildlike Behavior at its finest.Arial: Dark is my demon. He's going to be my husband, and only I'm going to have his mark over my heart. Do you hear me? If you kiss him, I'll claw off your lips. If you touch him, I'll cut off your hand. If you take his seed into you, I'll carve it out of you. He belongs to me. Everything about him is mine.
Nightmare Fuel / Rosario Vampire: Brightest Darkness