Sandel, an eleven-year-old boy, lives on the harsh prison island of Jananda, forming a close-knit group with Tonari and others after meeting her at age six. He openly shares dreams with them, aspiring to be a doctor. His upbringing amidst Jananda's violence fosters a pragmatic, cool-headed demeanor during crises; he calmly proposes medical solutions for severe injuries. Skilled with a bow, he uses explosive arrowheads against threats like Nokkers and lends his weapon to Fushi in combat.
On Jananda, Sandel witnesses Fushi's regeneration with initial shock, later turning to admiration. He participates in efforts to rescue Fushi from Hayase, joining his friends in a futile stand against Yanome guards. When a devastating second Nokker attack overruns the island, Sandel sees friends Oopa, Mia, and Uroy killed and puppeted. Amidst the chaos, he urges a guilt-stricken Tonari to survive and guides her toward the corpse dumping grounds, demonstrating resilience and loyalty. Post-attack, he chooses to remain on Jananda with Tonari, aiming to transform it into a place of redemption.
Forty years later, Sandel, now fifty-one, fulfills his dream as a doctor. He and elderly Tonari aid a village attacked by Nokkers, reuniting with Fushi and the Guardians, though neither recognizes them initially. Concerned about a Nokker within Guardian leader Hisame, Fushi requests Sandel perform surgery. Hisame drugs both Sandel and Fushi with morning-glory tea, rendering them unconscious. Sandel then tends to Tonari as she succumbs to fatal wounds from Hisame's Nokker. In her final moments, he comforts her with her favorite liquor and relays her dying wish for Fushi to show the forms of those he's encountered. Sandel observes these forms alongside Tonari, displaying quiet sorrow upon seeing Uroy, Mia, and Oopa.
After Tonari's death, Sandel consoles Fushi, emphasizing that grief is universal and urging him to end his isolation. He departs to inform Tonari's hometown of her passing, walking away while tearfully questioning aloud whether her regrets are finally resolved, underscoring his enduring care for Tonari and Fushi.