Shinobi Ai!

In an ancient fantasy world resembling feudal Japan, the nation's peace is upheld by demon hunters who work under cover of night to exorcise ferocious evil spirits. The young noble Fuyutsuki has just finished a mission with her retainer Shiranui, a kindhearted demi-human with mouths in his hands and a monstrous appearance. Though Fuyutsuki's ancestry requires them to keep their relationship secret, an early finish to their battle allows them a little bit of private time to get as freaky as they want in the great outdoors.

(Source: Irodori Comics)