From Illuminati-Manga:

Souten no Koumori is a oneshot by the mangaka of Fullmetal Alchemist.

It is the story of a shinobi, Henpukumaru, who awakens in an enemy's mansion. She is introduced to the future master of the mansion, Chiyozuru.

Chiyozuru is able to get the unemotional Henpukumaru to smile and react differently than she normally would.

But, one night, there is an attack by Henpukumaru's former allies...