Kanako Tsutsutori XL (Blood-C)

Kanako was a teacher for the main character but is later revealed to have tricked the MC by working with the antagonist. When she stops working with the antagonist, she gets attacked by a giant dog. The MC was able to save her but Kanako was untrusting of the MC, running away to the MC's mentor. Unfortunately, he had been turned into a monster, biting into Kanako's neck.