Black★Rock Shooter (OVA) Although she has slightly more screentime than Hiro, her presence is not essential to the story. She is first seen walking in on Mato as she was brushing her teeth. She was surprised to see her up so early and hoped Mato can keep up the routine. While cooking breakfast she tells Mato go wake up Hiro, and although Mato protests she does so after her mother reminds her she is the older sister.  When Yomi vanishes, mother Kuroi calls Mato and tells her to call her as soon as she leaves school as the police had come to question her on Yomi's disappearance. In this circumstance she could only comfort her daughter who cried for her friend.  Black★Rock Shooter (Anime) Appearance Mother Kuroi has short brown hair and blue eyes, her son bares a close resemblance to her. Her casual attire is a green shirt and jeans.  (Wikipedia)

It isn't the world you know. There wanders a girl with jet-black hair, ebony clothes, and a strikingly blue, glowing eye. Her name is Black&#9733;Rock Shooter. With a huge cannon in hand, she throws herself into a fierce battle... 
Meanwhile, in another world, middle-school student Mato Kuroi is drawn to Yomi Takanashi, her classmate with an unusual family name. Mato does her best to talk to Yomi, who just won't open up. Yu Koutari mocks her best friend Mato's efforts but at the same time, supports her. Mato also finds herself surrounded by other colorful characters at school, like the eccentric school counselor, Saya Irino, and the hot-blooded captain of her club club, Arata Kohata. 
Then, one day, the window of opportunity opens. After Mato sees that Yomi has her favorite picture book, "Li'l Birds At Play", they strike up a conversation and Mato is invited to come over to Yomi's house. There, they gradually break the ice, until a girl in a wheelchair appears. This is Yomi's best friend, Kagari Izuriha... 
Two worlds. Mato and Black&#9733;Rock Shooter. This is the story of their "pain". 
(Source: Nico Nico Douga)

Mato no Haha