Neptune Shoukaku is a character who appears during the Battle on Rakuyo arc of the anime series. He serves as the leader of the Fourth Division of the Harusame, a large space pirate syndicate. He is recognized as the strongest and most athletic assassin within the organization, known for his exceptional physical strength and mobility. His nickname is Neptune.

At his core, Shoukaku is a warrior who thrives on combat. He greatly enjoys battle and prefers to fight alone, relying on his own abilities rather than working with others. This warrior mindset drives his actions and makes him a dangerous opponent. His motivation is primarily tied to his role within the Harusame, where he carries out missions as a high-ranking enforcer.

In the story, Shoukaku is one of the three Mad Stars, the most feared members of the Harusame. He acts as an antagonist during the Rakuyo conflict, directly opposing the protagonists. His most notable confrontation is a one-on-one battle against Kotaro Katsura, a key member of the Joui faction. This fight showcases his combat prowess and his preference for single combat.

Information about his background beyond his position in the Harusame is limited. He does not undergo significant character development; he remains a straightforward antagonist focused on fulfilling his duties and engaging in battle. His key relationships are with his fellow Mad Stars and his adversaries, particularly Katsura.

His notable abilities include his superior physical conditioning and athleticism, which make him an agile and powerful fighter. He is described as a hitman who relies on brute strength and speed, making him a formidable presence on the battlefield.