Tadashi Daiba is the son of astronomer Dr. Daiba, murdered by Mazone invaders after witnessing their arrival on Earth. His father's death fuels Tadashi's vengeful pursuit, driving him to join Captain Harlock's crew aboard the Arcadia. Initially consumed by revenge, Tadashi gradually integrates into the crew, developing loyalty to Harlock and embracing their ethos of fighting for personal conviction. His evolution shifts from singular vengeance toward confronting moral conflicts, questioning violence's necessity and the Mazone's motivations. In select continuities, he forms a close bond with crewmate Kei Yuki that deepens into romance. Tadashi often anchors the narrative; his vulnerability draws exploitation by foes like the Mazone witch Aman, who captures him to lure Harlock into traps, notably during "Mystery of the Arcadia". Trauma from his father's death resurfaces, shaping his decisions and interactions. His presence fluctuates across adaptations: absent in "Arcadia of My Youth" and "Endless Orbit SSX", he resurfaces prominently in "Endless Odyssey" as Harlock reunites with him to combat the Noo entity after Earth's disappearance. He does not appear in the 2013 film "Harlock: Space Pirate".