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)