Space pirate captain Harlock operates in the late 30th century, primarily from the starship Arcadia. His background includes military service for Earth, where witnessing the government's betrayal led to disillusionment and his turn to piracy as rebellion against corruption and apathy. This origin, depicted in "Arcadia of My Youth," shapes his individualist philosophy. He fights solely for personal convictions rooted in freedom and honor, stating he acts only for "something deep in his heart".

The Arcadia, central to his identity, was built by his closest friend, Tochiro Ōyama. After Tochiro's death from illness prior to most narratives, his consciousness integrated into the ship's computer, creating a persistent spiritual bond. Harlock often senses Tochiro's presence or emotional state across the void. He assumes a protective role over Tochiro's daughter, Mayu Ōyama, frequently serving as her guardian and moral compass, especially in the original 1978 series; her safety becomes a recurring motivation.

His distinctive appearance features a prominent facial scar, an eyepatch over one eye, and attire often incorporating the number 42—a Japanese cultural symbol of death reflecting his acceptance of mortality. He typically wears clothing adorned with a skull emblem. Despite opportunities to repair the scar or replace the eye, he retains them.

Across continuities, Harlock confronts various antagonists. In the 1978 series, he battles the Mazone, plant-based aliens seeking to reclaim Earth under Queen Lafresia. Lafresia, revealed human, is defeated by Harlock, who permits her departure. Other adversaries include the Illumidas occupying force in "Endless Orbit SSX," the god-like entities of "Harlock Saga" (a reinterpretation of Wagner's Ring Cycle), and the ancient evil Noo in "Endless Odyssey," where Harlock emerges from exile after Earth's disappearance to rally his crew.

His crew aboard the Arcadia includes consistent members: Miime, a blue-skinned alien from Jura and the last of her species, possesses psychic abilities and profound loyalty, often acting as his closest confidante; Kei Yuki, a human woman functioning as navigator/science officer who frequently exhibits combat skills; Tadashi Daiba, a young recruit seeking vengeance for his scientist father's murder by the Mazone, often an audience surrogate. Others provide support or comic relief: first mate Yattaran, a skilled mathematician obsessed with model ships; Chief Engineer Maji; ship's cook Miss Masu; and frequently intoxicated Chief Medical Officer Doctor Zero, accompanied by his cat Mi-kun.

Character dynamics reveal facets of his personality. His complex relationship with Emeraldas (or Emeralda), Tochiro's lover and Mayu's mother, involves mutual respect and occasional collaboration. He maintains a stoic, often aloof demeanor as captain, embodying an "Aloof Leader, Affable Subordinate" dynamic contrasting with his crew. Generally reserved, he demonstrates deep emotional responses to Tochiro's loss or threats to Mayu. He consumes potent drinks frequently without visible intoxication.

Later adaptations introduce variations. The 2013 film grants him immortality intrinsically linked to the Arcadia's dark matter engine, enabling instant regeneration. "Gun Frontier" presents an alternate Old West version as a gunslinger aiding Tochiro Ōyama in locating missing Japanese immigrants, showcasing a more comedic and less central role. "Cosmo Warrior Zero" reframes him as a hunted fugitive pursued by the protagonist Zero.

Core traits remain consistent: an unwavering commitment to personal honor, willingness to defy corrupt authority, fearlessness facing death, and a protective instinct toward the vulnerable, especially Mayu. These define his enduring presence across manga, television series, films, and OVAs.