Emeraldas, a La Metalian from the planet La Metal, is the older daughter of Queen Promethium and Dr. Ban. Her mother's transformation into a machine entity to control La Metal ignited significant familial discord. Emeraldas developed fierce opposition to the Machine Empire, rejecting machine body conversion at every opportunity, even when facing death. This stance reflected her determination to preserve her organic identity and personal flaws, like the scar on her left cheek, which she kept as a mark of vulnerability. Her relationship with her twin sister Maetel grew strained over their differing reactions to their mother's actions, though mutual care remained.

As captain of the pirate vessel *Queen Emeraldas*, she forged a formidable reputation across the galaxy as an independent and feared figure. She adhered to a strict personal code, targeting only those she deemed cruel or unjust, aligning her methods with allies like Captain Harlock. Despite her imposing presence, profound loneliness defined her existence, driven by her choice to remain untethered and continue her cosmic voyage. Her interactions were often limited, yet she showed interest in individuals demonstrating unwavering dreams against impossible odds, such as Hiroshi Umino. To him, she gifted the Cosmo Dragoon, a weapon once owned by her husband, Tochiro Oyama.

Her romantic bond with Tochiro Oyama began with indifference but deepened into a profound connection upon recognizing their shared dedication and hopes. This relationship offered her new perspectives on life and human connection. However, their individual dreams prevented a lasting union, and Tochiro's death later intensified her isolation. This loss also complicated her relationship with their daughter, Mayu Oyama, whom she left on Earth and rarely contacted due to the painful memories associated with Tochiro.

During the Illumidas occupation of Earth depicted in *Arcadia of My Youth*, she joined the resistance. In this conflict, an Illumidas officer named Murigson inflicted her facial scar with a glancing shot. Despite initial neutrality concerns tied to her merchant license, she contributed her ship to the rebellion, assisting Captain Harlock and Tochiro and protecting refugees. These actions solidified her alliance with them, though Earth's government later exiled the trio as outlaws.

In *Endless Orbit SSX*, designated "X-00001" by the Illumidas, she operated alongside Harlock and Tochiro but with minimal direct involvement. Her appearances were sporadic, often involving rescue from Illumidas traps rather than driving the narrative. Her dynamic with Tochiro lacked development, omitting exploration of their romantic history, future, or their daughter Mayu. The series did not reconcile her portrayal with other media, such as her bedridden, near-death state due to illness in *Galaxy Express 999*, which contrasted her combat-ready depictions elsewhere.

Across various OVAs and spin-offs, character details shifted. The *Queen Emeraldas* OVA presented an alternative origin for her scar, attributing it to a sword duel instead of a gunshot. It emphasized her mentorship of Hiroshi Umino, showcasing her support for those pursuing incorruptible dreams. Inconsistencies persisted regarding her physical condition and combat capabilities, without addressing her *Galaxy Express 999* illness. Her broader appearances within the Leijiverse, including *Harlock Saga* and *Cosmo Warrior Zero*, failed to establish a unified timeline, further complicating her backstory and relationships.

As a La Metalian, she possesses an assumed long lifespan and formidable combat skills, particularly mastery of the Gravity Saber and quick-draw Cosmo Dragoon techniques. She is also a skilled navigator, familiar with obscure cosmic routes. When bedridden, she demonstrated the ability to remotely command androids resembling herself, maintaining control of her ship.