Rokurou Okajima, also known as "Rock," is the protagonist of the series. He was a Japanese salaryman for Asahi Industries in Tokyo until he was taken hostage by the crew of the Black Lagoon. Rock joined the Lagoon Company after his department chief, Kageyama, abandoned him to cover up a smuggling operation that Rock had unknowingly been part of. Despite facing danger from both allies and enemies, Rock remains humble and mild-mannered, often shocked by the brutality of Southeast Asia's criminal underworld. Even though he is now a pirate, he still wears his tie, short-sleeved dress shirt, and dress pants, clinging to his business persona. Preferring words over weapons, Rock has wondered if he might be experiencing Stockholm syndrome since joining the Lagoon Company.
Within the Lagoon Company, Rock handles account management, diplomacy, negotiation, interpretation, and occasional errands. His background in the resource investigation department at Asahi Industries makes him skilled in geology. He is also bilingual in Japanese and English and familiar with other languages such as Spanish, Romanian, and Russian. Rock’s high tolerance for alcohol comes from his previous lifestyle as an ambitious salaryman. His earnest personality has attracted the interest of several figures in Roanapur, including Balalaika, Mr. Chang, Rowan "Jackpot" Pigeon, Yolanda, Eda (who develops a crush on him), and young Garcia Lovelace, whom he trusts deeply. Rock serves as the heart of the Lagoon Company, balancing his more cynical teammates and bringing light to Roanapur's darkness. Dutch believes the company is complete with Rock not holding a gun, stating that his bullets would likely ricochet back at them.
Rock is one of the "luckiest" characters in the series, surviving everything from being held hostage to being shot at. So far, he has endured the wrath of Revy, Balalaika, Roberta, and Sawyer. Mr. Chang personally appoints Rock to help Garcia Lovelace, despite the Lagoon Company's disapproval. Over time, Rock begins to enjoy his life as a pirate and becomes more comfortable with corruption. In a conversation with Fabiola, he reveals a darker, ruthless side influenced by Revy and Roanapur's environment. Benny even notes that there are aspects of Rock that scare almost everyone in Roanapur.
Although Rock has a family in Tokyo, he does not seem closely connected to them due to their somewhat indifferent attitude. It was revealed that he did not pass his college entrance exams on his first try, causing his family to lower their expectations of him. He has a father, a mother, and an older brother who succeeded academically and secured a job in the Japanese government. According to the manga, Rock was born in the 49th year of the Showa period, which corresponds to 1975-1976.
