Lieutenant Junior Grade Rokkaku (ロッカク中尉, Rokkaku Chūi) was one of Axe-Hand Morgan's officers at his East Blue base. When Rokkaku refused Morgan's order to kill Rika, Morgan killed Rokkaku with his axe-hand (he was knocked into a coma in the English anime, with Morgan using the blunt end of his axe-hand).
His name apparently comes from a villain in Romance Dawn v.2, a prototypical One Piece manga. This is the only resemblance between the characters. Interestingly, Rokkaku (lit. Hexagon) has a face shaped like his namesake.