Black★Rock Shooter (OVA)
Despite the little screentime the interaction between Hiro and Mato seems to be the typical stubborn sibling relationship. When Mato's mother tells her to wake up Hiro, Mato moans in protests but does so, stating that Hiro never listens to her when she tries to wake him up. Her mother simply says Onee-chan, a tone reminds Mato that she is his big sister. Mato's method to wake up her brother was to open the door and give the order that school started that day, when Hiro doesn't respond she states that its not her fault if he is late and upon leaving gives him the raspberry.
After Mato leaves, Hiro awakes (somewhat) and stares at his alarm clock, only to collapse into a sleepy heap.

Black★Rock Shooter (Anime)
Hiro continues to have a very small role, occasionally bothering his sister. He doesn't like her sister speak to him in public because he feels embarrassed. Despite he doesn't have a good relationship with his sister, he likes her a lot.
In episode 7, Hiro is asked what does he thinks of his sister. His answer was: "¿My sister? She is always distracted, loses things very fast and is very annoying... but I think that my sister is a good person.

Appearance
Hiro, like his mother, has short brown hair and blue eyes. He is seen wearing a blue rugby shirt and dark green shorts. Naturally in being a little brother, his short stature is more apparent than Mato, during dinner he is sitting on cushion as to be level with the table.

(Wikipedia)