DARKER THAN BLACK: Kuro no Keiyakusha - Gaiden

Darker than Black: Origins

Fleeing from the consequences of his decision at the Hell's Gate, superpowered Contractor Hei and his companion Yin take refuge in a quiet inn, adopting the guise of a married couple in order to not draw suspicion. In an attempt to recover from recent events, Hei befriends the inn's other guests. He discovers that one of them is a fellow Contractor tasked with killing him. Their resulting encounter spells disaster for both Hei and Yin, who are forced to fight for their lives and grapple with the emotional wounds sustained in their previous life together.

(Source: MAL Rewrite)