Haruo Sakaguchi, a graduate of Shishiku Academy, serves as a young leading subordinate among the Onigashima organization's henchmen. His personality is explicitly characterized as perverted across source materials. Functionally, he acts as the primary caretaker for Onigashima Hikaru, undertaking all responsibilities under "Hikaru's orders" with unwavering dedication – a loyalty consistently met with disgust by Hikaru himself.

He holds the significant distinction of being the sole individual outside the twins privy to the identity switch between Hikaru and his twin sister, Hinako Nakayama. Consequently, he actively facilitates and maintains their deception, managing necessary family arrangements and formalities to ensure the switch remains undetected. This critical role demands ongoing logistical support and cover for both twins as they navigate their respective environments.

His history includes training and education at Shishiku Academy, an institution known for its delinquent student body, prior to his current position within the Onigashima yakuza group.