Zaryusu Shasha, a lizardman of the Green Claw Tribe, is the younger brother of its chieftain, Shasuryu Shasha. As a traveler who explored the outside world, he earned respect but held no formal authority within his tribe. His status rose significantly after acquiring Frost Pain, one of the Lizardmen's Four Treasures, by defeating the chieftain of the Sharp Edge Tribe. During his travels, he learned aquaculture and introduced sustainable fish farming to the Green Claw Tribe, enhancing their food security. He adopted a four-headed hydra named Rororo, who became his loyal companion.

Facing annihilation by Nazarick's forces led by Cocytus, Zaryusu advocated uniting the five lizardman tribes—Green Claw, Red Eye, Dragon Tusk, Razor Tail, and Small Fang—to resist. Acting as an envoy, he first visited the Red Eye Tribe. There, he met Crusch Lulu, the albino priestess and acting chieftain, immediately proposing marriage; though initially dismissed, they formed a partnership. At the Dragon Tusk Tribe, Zaryusu dueled Chieftain Zenberu Gugu, winning using Frost Pain's ice abilities and securing their alliance. The united tribes formed the Lizardman Alliance, led by Shasuryu.

In the initial battle against Nazarick's undead army, Zaryusu, Crusch, and Zenberu confronted commander Iguva=41. Rororo sacrificed himself to create an opening, allowing Zaryusu to deliver the killing blow. After this victory, a second overwhelming force led by Ainz Ooal Gown arrived. Recognizing resistance as futile, Zaryusu and Shasuryu negotiated surrender. Ainz demanded a duel against Cocytus instead, offering freedom if the Lizardmen won. Before the duel, Crusch, fearing Zaryusu's death, requested he father her child, which he did. During the fight, Shasuryu died creating an opportunity for Zaryusu to strike Cocytus, though both brothers perished. Ainz later resurrected Zaryusu as part of a magical experiment, solidifying Zaryusu's view of Ainz as godlike. Zaryusu pledged loyalty to Nazarick, and his brother and Zenberu were subsequently revived at his request.

Joining Nazarick, Zaryusu trained Hamsuke in martial arts within the Great Tomb and observed her combat trials. He later returned to the Great Lake region, where Crusch gave birth to their albino son. When Ainz offered him a role in E-Rantel to promote interspecies harmony, Zaryusu declined to stay with his family, suggesting other lizardmen instead. Ainz gifted him adamantite chainmail commemorating the child's birth. Zaryusu continued collaborating with Nazarick, assisting investigations into rising lake temperatures, participating in ceremonies like the Sorcerer Kingdom's National Foundation Day, joining aquatic combat training for elves, and supporting resource-mining initiatives. He leveraged his status to foster unity among the tribes under Nazarick's oversight.