The young Ianna had a great liking for dogs and most other creatures, even those who most people were disgusted with. One evening she risked her own life to rescue a large mosquito from the web of a Silverspider, and the mosquito bit her for her pains and flew away. Scratching the bite on her cheek, she went home wondering why she bothered to rescue it in the first place. That night, she swiftly became ill as she lay on her bed, too ill even to cry out for help. She thought she was dying, too weak even to swat away the mosquito that buzzed around her head.
Settling by her ear, instead of biting her it began to speak to her in words that she heard within her head. It told her that it was one of the component parts of a Gichigumi, a were-mosquito, and that out of gratitude for her saving it from a sticky death at the spider's mandibles, it had decided to turn her into a Gichigumi too. In a while, it said, she would grow stronger again and by concentrating when it was night, she could turn herself into either a swarm of small mosquitos or a large, dog-sized one. Then it flew away, and within hours she found it had indeed been telling her the truth.
Disgusted at first, she decided not to use her powers ever, but when her sister was brutally raped by a couple of noblemen, who were too highly placed to proceed against in the court system, she visited their houses in the night, turned into a swarm to get inside, then turned into a dog-sized mosquito, pierced their mosquito nets that they had over their beds, and then pierced their flesh without waking them. She knew if she drunk them dry then there would be a witch hunt for whoever or whatever had done it, so she only drunk a small amount of their blood but infected them with disease before flying away back home.
Within a few days they had died painfully, and with them being nobles, the City Guards carried out a proper investigation. First they brought in a mage to check for traces of magic that could be traced back to the caster, but found nothing. Then they checked the bodies for wounds, but all they found was a large bug bite that had been clearly been scratched open. Finally a check for poison found only that they had died of disease.
Over the next few years, a small number of people that have threatened Ianna or her family have died in the same way, seemingly of disease caused by a bug bite. There was a loanshark who tried to extort money from her father, one of her sister's husbands who was brutal towards her sister, and a noble who made the mistake of threatening her family with eviction.
She was recently left a large sum of money by somebody who she tended in his last days after he came down with disease, again from a bug bite.
On the surface, she seems like a perfectly pleasent young woman, and does not need to feed in her vampire form, provided that she has got enough food in her human form.
Possible Plot hooks:
:One of the friends of the PCs has been killed in such a way, the PCs are investigating the death.
:After a famine in the region has broken out, people are turning up drained dry of blood.
:The PCs come under attack from her. Can they fight her off, and then discover what has happened?