Mahiro is the only female Iga Ninja. she has an older brother named Muramasa and a sister named Mayumi. Mayumi was attacked by Chinmei and was killed giving Kyo the muramasa, but Mahiro only saw Kyo with a bloody sword and her dead sister (in the anime, a Kenyo attacked her as she gave Kyo the sword and asked him to kill her with it - Mahiro saw Kyo killing her sister, but she didn't know Mayumi asked Kyo to kill her). She decided to take revenge.
Four years later (she's 19 now), she is a trained ninja and sets out to kill Kyo. When she, Benitora and Sasuke catch up to the third gate, Mahiro overhears Chinmei telling the story about him and Kyo's meeting. She discovers that it was Chinmei, not Kyo, who killed Mayumi (in the anime, when she fought him and discovered the truth, she was going to kill herself, but Kyo stopped her. He told her that she should live on to avenge her sister).