Lyza, the legendary White Whistle delver dubbed "Lyza the Annihilator," serves as the narrator and mother of Riko. Riko was conceived and born during an Abyss expedition in the fourth layer. Though stillborn, Lyza and her mentor Ozen revived Riko using the Curse-Repelling Vessel relic. This occurred near the conclusion of the Unheard Bell retrieval mission, which claimed the lives of Lyza’s husband Torka and numerous expedition members. Upon surfacing, Lyza entrusted infant Riko to Orth’s Belchero Orphanage. When Riko turned two, Lyza commenced her Last Dive into the Abyss’s depths and vanished.

Ten years after her disappearance, artifacts from Lyza’s final dive emerged, including her White Whistle, personal notes, and a message for Riko: "At the Netherworld’s bottom, I’ll be waiting." This discovery ignited Riko’s journey into the Abyss. Lyza’s notes and sketches depicted a humanoid robot resembling Reg, indicating prior awareness of his existence. Her narration frames the story, contextualizing the Abyss’s enigmas and foreshadowing pivotal events.

Flashbacks and revelations expand Lyza’s background across the series. Her Unheard Bell expedition entailed clashes with foreign delvers, leading to heavy casualties. Her bond with Ozen, outwardly stern, harbored profound mutual respect; Ozen vowed to enable Riko’s descent if possible. As a pioneering White Whistle explorer, Lyza commands reverence and speculation among Orth’s delvers. Her unresolved fate and motives for summoning Riko to the Abyss’s depths propel the core narrative.

In the anime adaptation, Lyza’s narration punctuates critical scenes, elucidating the Abyss’s perils or echoing her notes. Her voice accompanies sequences involving her relics, such as the unearthing of her message. The storyline progressively uncovers her impact on characters like Bondrewd, who cites her discoveries in his experiments. Her legacy intertwines with the Abyss’s broader lore, including the Curse and relics.