Sin

The protagonist, Kai Kishou, has moved home many times since he was a child.
Because of all these goodbyes he has become indifferent to people, but when making arrangements at a new school he suddenly encounters a girl being attacked by strange men.
These strange attackers are not normal humans.
The targeted girl, and the beautiful swordswoman who protects her.
The world he has been thrust into will completely change Kishou's destiny.