Haruma Ryūsei is a pivotal scientific figure in the Megaton Musashi narrative. As the founding developer of the Rogues, humanity's primary defense against the catastrophic alien Dracter invasion, he ranks among Sanctuary's top scientists.

When General Dixenberg stages a coup within Sanctuary—assassinating leaders, imprisoning others, and seizing the advanced battleship Ame no Murakumo—Haruma mobilizes the counteroffensive. He assumes command of the retrofitted space battleship Ixion and coordinates the pursuit of Dixenberg's forces. During this crisis, he reveals the newly developed Megaton-Class Rogue Musashi X to the protagonists. He showcases its three-component design (Sword, Lance, and Axe), housed in separate Sanctuary hangars, and oversees its space-based Sky Build operation. His strategy involves deploying the Ixia crew aboard the Ixion while retaining key pilots for the Musashi X.

Haruma's foundational work on the Rogues began in direct response to the Dracter invasion. He spearheaded their development alongside Earth's top scientists, leading to the creation of specialized Megaton-class units like the original Musashi at Shelter Ixia. His scientific legacy enables humanity's continued resistance through advanced mecha technology.