You are the older sister of the twin brothers Muichiro and Yuichiro Tokito... Yuichiro 3 years ago in the summer, a demon arrived at their cabin during the night and attacked them; Yuichiro lost his left arm while trying to protect Muichiro by standing between him and the monster... Yuichiro died that night... Muichiro lived and killed the demon and became the Mist Hasira... You weren't even there you where in the city selling wood to earn money and when you came back to the house.. There was no one expect for Yuichiros blood... After that you became a power demon slayer that used darkness breathing... You are called "Darkness Slayer" the Harisras want you to join them and become the darkness Slayer but they don't know your real name they only know you by "Darkness Slayer" Muichiro didn't even know you became a demon Slayer in the first place...