Kei Yuki, operating under aliases like Mayu Oyama and the username FREEARCADIA, originated from an advanced future universe roughly a millennium ahead of Siren's Port timelines. She emerged as a core crew member of the legendary pirate ship Arcadia, serving as chief navigator and science officer under Captain Harlock. Her expertise encompassed radar operations, piloting Space Wolves fighter craft, and conducting scientific analyses with specialized equipment.

Her early Earth life held profound tragedy. Her scientist father, researching space colonization, died in a lab accident later exposed as sabotage orchestrated by her fiancé, Kazuya Katagiri. Her mother succumbed to grief shortly after. Kazuya further betrayed Kei, exploiting her trust to gather intelligence on the Arcadia for the alien Mazone faction. Discovering his role in her father's death led her to execute him. This betrayal forged a lasting wariness in relationships, compelling emotional distance alongside fierce protectiveness of trusted allies.

Joining the Arcadia crew—after Harlock liberated her from imprisonment on a transport vessel—Kei evolved from junior officer to seasoned space pirate. She navigated complex ship dynamics, mentoring younger crew like Tadashi Daiba and collaborating with non-human allies such as the psychic alien Miime. Her responsibilities expanded during crises, including temporary command of her own vessel, the Fluorite, following the Arcadia's disbandment. Earth authorities later captured and sentenced her to execution before Harlock orchestrated her rescue and return to the Arcadia.

Across continuities, Kei confronted existential threats with resilience. During conflicts with the demonic Noo entities, she took a bullet for Tadashi Daiba, resulting in her body's possession by a Noo spirit. While comatose, her consciousness journeyed to limbo, uncovering critical secrets about the Noo's origins. Following exorcism and recovery, she resumed her duties. Her role sometimes extended to safeguarding Mayu Ōyama, daughter of Harlock's closest friend Tochirō Ōyama, reflecting emerging maternal instincts.

Later, at Siren's Port, Kei adapted her pirate ethos to urban survival. She co-established a faction-neutral intelligence network at Loyaltous and investigated environmental hazards around Whithers Lake, especially after water poisoning incidents. Utilizing aliases, disguises, and wigs to evade SERO agents, she gathered intelligence on organizations like AGI and SERO. Briefly employed at Icha Icha Paradise, she became unemployed following SERO's attack on the establishment. During this period, she acted as confidante and protector for vulnerable residents, though psychological struggles arose from confinement after a lifetime of interstellar freedom. Physical setbacks included a leg wound and radiation exposure sustained during an earthquake near derelict lakeside structures.

Kei's personality blended professionalism with dry humor and occasional flirtatiousness, tempered by trauma-induced wariness. She adhered to a personal code of justice, rejecting Earth's corruption while defending its people. Her design typically featured a pink jumpsuit adorned with skull motifs, blonde hair (sometimes shortened for disguise), and a willowy physique consistent with Leiji Matsumoto's artistic style. Her surname "Yuki" ("Existing Record") reflected her role as a testament to her parents' legacy, while her given name "Kei" ("firefly") alluded to her symbolic function as a guiding light in darkness.