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Sayla Mass, born Artesia Som Deikun, is the daughter of Zeon Zum Deikun and younger sister of Char Aznable. After her father's death and the Zabi family's rise to power, she and her brother were secretly taken to Earth by Jimba Ral. Adopted by the Mass family to avoid persecution, she changed her name and lived on Earth before relocating to Side 7, where she served as a medical volunteer.

During the One Year War, she evacuated to the SCV-70 White Base following a Zeon attack on Side 7. While searching for survivors, she faced Char Aznable and recognized him as her brother. She joined the White Base crew as a communications officer and later became an alternate pilot for the RX-78-2 Gundam after an unsanctioned attempt to confront Zeon forces for information about Char, which resulted in disciplinary confinement despite acknowledgment of her piloting skills. She transitioned to piloting the G-Fighter (Core Booster in compilations), demonstrating proficiency and emerging Newtype abilities in combat.

Her relationship with Char remained complex. She refused his offers to abandon the Federation Forces during encounters at Jaburo and Texas Colony, where he revealed his revenge plot against the Zabis and provided her with gold bars. She requested Captain Bright Noa distribute the gold among the White Base crew, whom she viewed as family. During the Battle of A Baoa Qu, she crash-landed and intervened in Char's duel with Amuro Ray. Char urged her to leave with Amuro, facilitating her escape from the fortress.

In the Gryps War era, she monitored her brother's activities as Quattro Bajeena and later met Kai Shiden to discuss Char's disappearance. During the First Neo Zeon War, she rescued Leina Ashta and revealed to Bright Noa that she lived as an investor, expressing concerns about Char's ambitions endangering peace. By U.C. 0095, she managed a foundation for war orphans and discussed Char and Amuro's shared connection to Lalah Sune with Stephanie Luio, noting Lalah's death profoundly impacted both men.

Her relationship with Amuro Ray evolved from camaraderie to romance in some adaptations. Novelizations depict them becoming intimate during the war, with Amuro telepathically expressing love for her during his death. Manga adaptations like *Mobile Suit Zeta Gundam Define* show them maintaining a romantic relationship post-war.

In the Cucuruz Doan's Island incident, she piloted a Core Booster during a rescue mission for Amuro Ray. Her aircraft sustained heavy damage in an engagement with the Southern Cross team, forcing a crash-landing on the island alongside Sleggar Law.

Post-war, she established a foundation caring for orphaned children, including Jill and Milly Ratokie. This humanitarian work demonstrated her rejection of Zeon's political legacy despite her lineage.