In "Adieu Galaxy Express 999", Harlock appears as a supporting character aiding protagonist Tetsuro Hoshino's journey. He rescues Tetsuro when stranded at La Metal station, demonstrating his recurring role protecting those opposing oppressive forces, consistent with his persona as a space pirate operating outside Earth's authority.

Across the broader Leijiverse, Harlock is established as a descendant of the Harlock lineage, with Phantom F. Harlock III being the prominent incarnation. He inherits the title and mantle from ancestors Phantom F. Harlock I and II. The character maintains connections to key figures like his cousin Emeraldas and another cousin, Maetel, central to the Galaxy Express 999 narrative. His background includes military service for Earth followed by disillusionment with its corrupt governance, leading him to become an outlaw. His iconic facial scar and eye patch stem from combat injuries sustained during his conflicts.

Harlock commands the starship Arcadia with a crew of rebels and outcasts. His values emphasize individual freedom and resistance against totalitarian regimes, human or alien. He operates under a personal code of honor, often protecting Earth despite its government branding him a criminal. His motivations originate from a promise to his deceased friend Tochiro Ōyama, the Arcadia's creator, to safeguard humanity's future.

Across various narratives, Harlock remains consistent in rejecting authority and committing to interstellar justice. His intervention in "Adieu Galaxy Express 999" exemplifies his role aiding protagonists who challenge oppressive systems within Leiji Matsumoto's interconnected universe. His appearance reinforces his thematic function as a symbol of rebellion and hope.