This is... Wow, this is something else. Miwako is a great antagonist, she's honestly the smartest character in the whole story, even smarter than her sister who seems to have an inferiority complex considering the way that she picked up the scraps of Miwako's old errand boy and tried to get Souichi to fall for her instead. But the protagonists... Or, I suppose, "anti-heroes" is a better term for them, they're just really stupid. I get that they're teenagers, but they're just stupid.
Hirota is moderately sympathetic, seeing how he at first tried to fight the whole transformation, and only gave in in order to save Chika. Then, when Souichi tried to talk to Chika in order to call out Hirota, Hirota had to put on the new mask in order for him to be good enough to beat Souichi, and ultimately, to protect Chika as well. Except, the mask is clearly altering his humanity, which ended up pushing Chika further and further away.
Souichi is the least sympathetic character, in my opinion, because it's not like he started out innocent and was twisted, even his backstory about being abused and neglected isn't enough to justify him literally being a murderer. Even as a human, Souichi straight up just brought people to Miwako for the slaughter like some psychopath. I don't care if he was bullied, he straight up killed innocent people. Literally the only reason why Souichi brought Hirota to Miwako in the first place is because Souichi was jealous of Hirota's happiness. His story isn't the one of one-upping his bullies, he's just a psychopath that had a fetish for feeding Miwako blood, that's it. His whole turn of face as a character after Chika was being nice to him is really not about Chika, it's about Souichi wanting to feel "special," which is just...gross.
Chika is so one-dimensional that she can't even come up with a better reaction to seeing Souichi literally becoming a monster and trying to rape her than "You're a good person!" C'mon, Chika, can't you at least pretend to have some personality? At least she'll hopefully get a little bit of a better character now that she can stop being the innocent-shoujo-girl.
Miwako's sister is so much of a deus ex-machina that she's basically just a worse Miwako and with lower self esteem. All she's done is 1. bring Souichi back to life, 2. bring Chika back to life. Otherwise she's just been annoying and super in the way.
Miwako herself, man, she's so evil that she almost circles back around to looking innocent. What a master manipulator, though it seemed that she didn't expect Souichi to come back to life (or did she expect her inferiority-complex-sister to bring her own minion back to life?), but she used every opportunity she got to get exactly what she wants. Which seems to be to have Hirota lose everything in his human life in order for him to become dependent on Miwako. Except she can't just do it outright, because then Hirota would hate her. No, she has to manipulate the situation, make him first think she's helping him, that she's trustworthy, that she'll protect him. It's smart that Miwako didn't use Souichi, because someone with nothing to lose could just as easily attempt to cast her aside at some point in the far future, psychopaths don't make great henchmen because they're too self-absorbed. Look at how Souichi has treated Miwako's sister, as soon as the sister did something that Souichi didn't like he totally betrayed her. Miwako is instead taking her time working all of the charms she has onto Hirota, until he trusts her implicitly and feels indebted to her. Look at the way that Hirota treated Chika, he literally gave up everything, being human, or at least dying before he could truly become a monster, because Hirota wanted to make a deal to protect Chika. Miwako wants that kind of loyalty for herself, and she's orchestrated everything perfectly for that to happen... At least, until Chika had to become a party pooper and become a vampire.
Seriously, Chika becoming undead basically just makes this whole "ntr" thing even more annoying. Because, unless I'm mistaken and this is somehow a part of Miwako's plans, now Hirota might be more inclined to act as Chika's familiar which is just... Wow, what a disappointing outcome for such an originally great concept.