Kohaku was born in Ishigami Village roughly 3,700 years after the global petrification event. She is the younger sister of Ruri and second child of Kokuyo, descended from the unpetrified humans of the modern era. When Ruri fell seriously ill, Kohaku dedicated herself to daily care, carrying heavy loads of hot spring water, revealing early determination and physical prowess. As Ishigami Village's strongest warrior, Kohaku possesses exceptional strength, agility, and keen eyesight. Her combat skills enable her to swiftly dispatch multiple opponents, while her visual acuity proves strategically valuable, such as spotting oil fields from a hot air balloon. She exhibits a tomboyish personality and reacts negatively to comparisons to animals like gorillas or lionesses, though she tolerates such remarks from close allies like Senku. After encountering Senku, Kohaku becomes one of his earliest and most loyal supporters in the Kingdom of Science. Witnessing his bravery and scientific ingenuity, she invites him to Ishigami Village. Her initial distrust of outsiders evolves into unwavering loyalty, particularly after Senku develops a cure for Ruri. Kohaku frequently assists in gathering materials, enlists villagers for labor, and participates in critical missions against adversaries like Tsukasa and Hyoga. During the battle with Hyoga, she narrowly avoids fatal injury due to Gen's intervention and later coordinates villager safety during an attack. Kohaku plays pivotal roles across multiple arcs. In the Village Games, she supports unconventional tactics to secure resources for Ruri's medicine and intervenes to protect Suika from captivity. During the Communications Arc, she enthusiastically backs Senku's cellphone project and contributes her father's shield for gear production. In the Age of Exploration Arc, she joins Ryusui Nanami's voyage, her annoyance surfacing when Senku teasingly calls her a lioness. Her eyesight aids in locating resources, and she participates in early farming initiatives and culinary training under Francois. On Treasure Island, Kohaku infiltrates a local tribe's harem alongside Amaryllis and Ginro to gather intelligence and secure the petrification device. She devises covert communication methods with Senku, including using "Lab" as a distress signal, and shares a tactical kiss with him to avoid detection. Petrified by the enemy, she is later revived and reunites with her allies. Following a second worldwide petrification event, Kohaku is depetrified seven years later and tearfully embraces Suika, acknowledging her friend's growth. Kohaku's relationships are integral. She shares a close bond with Ruri, driven by a fierce desire to protect her. Her dynamic with Senku blends deep respect, reliance on his scientific leadership, and occasional romantic teasing—she acknowledges finding him physically attractive and once proposed a betrothal. She mentors Chrome, supports Suika like a younger sister, and maintains cautious camaraderie with Gen despite initial distrust. Her interactions with Ryusui involve shared leadership during explorations, though she finds his behavior unsettling. Kohaku views Tsukasa as a formidable enemy initially but later sympathizes with his familial motivations after learning of his sister Mirai. She pursues a rematch against Hyoga, demonstrating persistent growth. Kohaku adapts to broader scientific endeavors over time, including radar construction and international voyages. After reuniting with the Kingdom of Science post-second petrification, she continues collaborating in global missions, such as the journey to South America and preparations for lunar exploration.