Lyza, often referred to as Lyza the Annihilator or the Sovereign of Annihilation, is a legendary White Whistle delver and a central figure in the lore of the Abyss. She is the mother of Riko, the story's protagonist, and was the apprentice of Ouzen, another White Whistle. Lyza is remembered for her immense combat ability, having defeated countless dangerous creatures and foreign delvers, earning her fearsome title. She is also credited with discovering a large number of Special Grade Artifacts, marking her as one of the most accomplished delvers in history.

In personality, Lyza is described as fiery, assertive, and unapologetically impulsive. She loves parties, drinking, and fighting, and is known for her cheerful, optimistic, and carefree demeanor. She expresses her thoughts without a filter and acts on instinct, which often led others to view her as eccentric or even reckless. Despite this, she possesses a deep sense of loyalty and affection for those close to her, especially her mentor Ouzen and her daughter Riko.

Lyza's motivations are driven by an insatiable curiosity about the Abyss, a trait she shared with her daughter. From a young age, she was obsessed with exploring its depths, and she boldly approached Ouzen to demand she become her apprentice. Her drive and talent eventually elevated her to the rank of White Whistle. After marrying the Black Whistle delver Torka and giving birth to Riko under harsh conditions inside the Abyss, Lyza faced personal tragedy when Torka was killed in a conflict with foreign delvers and Riko was stillborn, only to be revived by a rare relic. Despite her love for her child, Lyza believed that staying with Riko would rob her of the chance to pursue her own adventures. When Riko was two years old, Lyza undertook her Last Dive into the Sixth Layer of the Abyss, a point of no return, and has been missing ever since. Her final notes and a letter addressed to Riko were later retrieved by Ouzen, sparking Riko's own journey into the depths.

Lyza's role in the story is primarily as a legendary figure whose legacy and absence drive the narrative. Her disappearance motivates Riko's quest to find her, and her discoveries and writings provide crucial knowledge about the Abyss. Key relationships include her mentorship with Ouzen, which evolved into a bond of mutual respect and friendship; her role as a mother to Riko, whom she left behind to follow her calling; her brief marriage to Torka; her apprenticeship of Jiruo; and a mysterious connection to the robot boy Reg, who shares her catchphrase and has fragmented memories of sharing meals with her. Lyza's development is largely revealed through the recollections of others, showing a growth from a brash, determined child into a legendary but still unconventional White Whistle.

Notable abilities include her extraordinary combat skills, allowing her to defeat opponents other White Whistles struggled against. She was also highly resourceful in surviving and navigating the Abyss, as evidenced by her detailed survival notes and cooking advice. Her strength and will were such that she earned the title Annihilator, and she is remembered as one of the greatest delvers ever to descend into the Abyss.