Mayumi

Mayumi is Mahiro's sister and was married to Muramasa for a period of time after he escaped from the Mibu clan. Mayumi's dead body been found by Mahiro, Kyo was beside Mayumi, holding a bloody sword in hand, which made Mahiro believe that he was the killer.

[Spoiler] She was killed four years ago by Chinmei in manga and Genza in anime. However, Mahiro doesn't find out the truth until much later in the story. [/Spoiler]