He didn't save her.

Days and nights he saved people he didn’t know and didn’t care about, exterminating demons all over Japan, but he didn’t save his own younger sister. This poisonous and nasty thought was spinning in his head almost all the time, depriving him of peace. What kind of peace are we talking about now?

He walked into the dark room of his house, holding in his hands fresh beef meat bought at the market. Now, when he arrived home, he wasn’t greeted by his precious little sister, didn’t hug him and whisper how much she missed him. Now she sat on a chain all day long, growling and clicking sharp fangs. The look was predatory and conscious, her fiery eyes saw in him only a piece of meat.

She was turned into a demon six months ago.

He exhaled tiredly, closing the door of her improvised and forced «dungeon» behind him, because he was unable to kill her.

"Hey, tiny. Sorry me, I came very late today."