Kei Yuki, a human female combat commander, navigator, and science officer aboard the starship *Arcadia*, navigates a turbulent past marked by orphanhood stemming from government negligence. In one continuity, her scientist father perishes in a lab accident orchestrated by her fiancé, Kazuya Katagiri, whose subsequent betrayal allies him with the alien Mazone. After her mother’s death from grief, Kei is rescued by Captain Harlock from a prison transport, cementing her loyalty to him. Balancing professionalism with dry, flirty humor honed among the *Arcadia*’s crew, she channels a fierce sense of justice and devotion to Harlock, whom she credits for her freedom. Her dynamic with younger crewmate Tadashi Daiba shifts from friction to protective mentorship, climaxing in *Endless Odyssey* when she intercepts a fatal bullet meant for him and endures temporary alien possession. Proficient in spacecraft piloting, radar systems, and tactical prediction, Kei also wields a shamisen to uplift crew morale. Later narratives, such as *Dimensional Voyage*, expand her role as she combats Mazon threats and crosses paths with figures from the broader Leijiverse, including *Galaxy Express 999* characters. Alternate timelines reimagine her origins—her father becomes a newspaper publisher in *Endless Orbit SSX*—yet her resilience persists. After briefly commanding her own pirate crew post-*Endless Odyssey*, she returns to Harlock, demonstrating leadership versatility. Later portrayals highlight a maternal dimension through her guardianship of Mayu Ōyama, daughter of Harlock’s late ally Tochiro. Clad in Leiji Matsumoto’s signature skull-adorned pink-and-black uniform, with a willowy frame and blonde hair, Kei’s subtle romantic undertones toward Harlock in early media gradually shift toward unwavering platonic allegiance in subsequent adaptations.