Kohaku is the younger sister of Ruri. She lives in Ishigami Village with her parents, Kokuyo and Kokusai, and is a member of the Ishigami Village High Priestess's Council. She and her family are tasked with preserving the Hundred Stories (Bai Wu Yu, Hyaku Monogatari) for future generations.
She is very close with her sisters and a strong advocate for their welfare. She also takes a liking to Senku when she first meets him. She is the leader of her village's efforts to help Senku restore her sisters health, as she has a debilitating illness that could kill her before she's even grown up.
Her senses are enhanced, including a sharper sight and hearing than most of her friends. She can also spot her enemies from a distance, and can find oil in crude aerial photos.
Despite her abilities, she often gets into trouble because of her impulsive behavior and her inability to keep her temper under control. She is a bit of a hot head at times, but she learns to calm herself and think more than one step ahead when she's with Senku.
Her loyalty is also shown early on, when she enters a fighting tournament in order to prevent her older sister from marrying Magma. She later becomes an ally of Senku's, especially when she realizes that he is the savior the villagers' ancestors speak of in their ancient stories.
She eventually becomes a valuable ally to the Kingdom of Science, but she continues to have lingering skepticism about Hyoga's actions, which she warns the group about. She also feels that Hyoga's proximity to Dr. Xeno seems to be a bad idea, as it can potentially affect the Kingdom of Science.
