Rei Yuki is the second mate aboard the starship Arcadia, monitoring radar during missions. She capably pilots both the Arcadia and smaller Space Wolf jets. She wears a distinctive pink uniform bearing the crew's skull and crossbones insignia, marking her place in the pirate organization.

Her parents' death due to Earth government negligence fueled her disillusionment and decision to join Captain Harlock's crew, aligning with his fight against oppression. Her initial interactions with crewmate Tadashi Daiba were marked by friction over his impulsiveness and clumsiness, though this relationship later evolved into mutual respect and closeness, becoming romantic in some storylines.

Within broader narrative continuity where Tadashi is absent, she becomes the sole prominent female crew member until Miime arrives. Some depictions show her harboring unrequited feelings for Captain Harlock, adding complexity to her shipboard relationships. Her steadfast loyalty drives her to fulfill duties as both a technical operator and combatant.

Her character development traces a path from a recruit disillusioned by Earth to a committed member of Harlock's resistance. The evolution of her relationship with Tadashi, when he is present, serves as a recurring arc, shifting from antagonism to partnership and sometimes romance. Her role consistently emphasizes technical proficiency and combat readiness, securing her position as a trusted officer on the Arcadia.