Hiro Kuroi  

, ヒロ

Black Rock Shooter OVA portrays Hiro and Mato's interaction as typical of siblings having a stubborn relationship. When their mother tells Mato to wake up Hiro, she uses the term "Onee-chan," which reminds Mato that she is his big sister. Mato tries to rouse her brother by opening the door and declaring school has started, but when he doesn't respond, she gives him a raspberry and leaves. Afterwards, Hiro awakens somewhat and looks at his alarm clock before collapsing into a heap. In contrast, Black Rock Shooter anime features Hiro as having a small role in the series, occasionally bothering his sister. He doesn't like her speaking to him in public because he feels embarrassed. Despite their strained relationship, Hiro likes Mato and considers her a good person. In episode 7, when asked about his thoughts on his sister, Hiro describes her as being easily distracted, losing things quickly, and being annoying. Both the OVA and anime depict Hiro with short brown hair and blue eyes. He wears a blue rugby shirt and dark green shorts, and during dinner he sits on a cushion to be at eye level with the table due to his shorter stature as a little brother.