Kokuyo held the position of Ishigami Village chief prior to Senku's arrival, having earned the role through victory in an earlier Grand Bout tournament. He is the father of Kohaku and Ruri. Physically, he is an imposing middle-aged man with significant musculature, blond hair tied in a ponytail, a long brown beard and goatee, and intense brown eyes. He typically wears a blue robe over a white undercloth, secured with a rope tied around his neck. Initially presenting as harsh and authoritarian, Kokuyo enforced village rules strictly, threatening to disown Kohaku for disobedience like entering the tournament against his wishes or bringing outsiders such as Senku into the village. He displayed little concern over Magma potentially becoming chief despite knowing Magma disregarded Ruri's well-being. However, underlying care for his daughters existed, particularly his worry over Ruri's pneumonia. He struggled emotionally with Kohaku's distance, exemplified when gifting her a commemorative shield crafted by Kaseki and feeling visible sadness as she readily offered it for Senku's inventions without sentimental hesitation. Significant development followed Senku curing Ruri's illness. After initially blaming Senku for Ruri's worsening condition and attacking him as a sorcerer, Kokuyo ultimately acknowledged Senku's leadership and formally recognized him as the new village chief upon Ruri's recovery. He grew to appreciate the Kingdom of Science's technological advancements, praising Senku for making survival enjoyable. Though no longer chief, he remained a co-leader focused on practical tasks like food gathering while Senku pursued scientific progress. Kokuyo retained the villagers' respect, demonstrated when they followed his commands during crises like Homura's attack. Kokuyo possessed exceptional physical strength and combat prowess, recognized as one of Ishigami Village's primary fighters and its strongest warrior in his prime. Skilled in swordsmanship, he wielded katanas against Hyoga's invasion and during assaults on locations such as the miracle cave. His leadership experience from years as chief enabled him to efficiently organize villagers during threats, evacuating seniors and children while directing others to confront invaders. Throughout the Stone Wars, he actively participated in battles, defending the village with scientifically crafted weapons. He later joined expeditions like the voyage to Treasure Island aboard the Perseus, expressing suspicion about the Why-man yet showing compassion upon learning Soyuz's traumatic past. Twice petrified on Treasure Island—first by Kirisame and again by Ibara—he was revived both times. During the global petrification event in South America, he instructed villagers to assume defensive positions before being petrified himself. Revived nine years later by the returning Kingdom of Science, he expressed joy at Kohaku's return. He later confirmed neither he nor other villagers constructed the protective wall shielding the petrified villagers, leading to the revelation that Sagara and Chalk built it. Kokuyo's practical mindset occasionally led to misunderstandings, such as believing tungsten was for throwing at enemies rather than scientific experiments or assuming jewels were solely for village defense. Villagers like Ruri and Kohaku attributed this to his older generational perspective.