Mukuro's perpetually gloomy younger sister. She has a fondness for Mugen that dates back to their days of piracy, though the feeling is not quite mutual. Koza isn't so much in love with Mugen as she is attached to him.

This stems from her chronic dependency—she simply can't bear to be alone, which explains why she has remained with Mukuro all this time, even though he clearly repulses her. She devises a scheme to get rid of Mukuro and leave with Mugen, but as Mugen rejects her advances, she replaces him with Shiren. She neglects to inform Mugen what Mukuro has in store for him. After the ship Mugen is on explodes, she manipulates Jin and Mukuro into fighting, which ends in Mukuro's death. As she is leaving with Shiren, Mugen, having survived the explosion, confronts them and cuts Shiren down with a series of lightning-quick slashes despite being rigorously weakened. Koza begs him to kill her. He pays her no heed, sick of her antics and furious beyond words at her betrayal; he departs, leaving her to suffer alone.
(Source: Wikipedia)