Kaseki is a primary artisan of Ishigami Village, an elderly man with a bald head, a very long and full white beard, and a short stature. Despite his advanced age, he possesses a surprisingly robust and muscular physique, which becomes especially apparent whenever he becomes deeply engaged in his work, a state that frequently causes his clothes to rip apart from his swelling frame.

In terms of background, Kaseki is a descendant of the astronauts who founded the village, and his given name is the Japanese word for fossil. From a young age, he was passionate about creating things, but in a community that placed a higher value on strength and combat prowess, his interests often led to him being looked down upon and isolated by his fellow villagers. Consequently, he spent much of his life as a somewhat solitary figure, channeling his energy into building much of the village's infrastructure, including its bridges and creating shields for its protectors.

Kaseki's personality is defined by his immense pride in his craft and his honest, straightforward nature. He is generally calm and unassuming when not provoked, but he harbors a boundless enthusiasm for any new challenge that tests his skills. Initially, he was distrustful of Senku Ishigami, viewing him as an outsider. However, his deep-seated passion for crafting was ignited when he witnessed Senku attempting to work with glass, a material he had never seen before. Unable to bear the amateurish attempts, Kaseki broke free from restraints to take over the process himself. From that point, he became one of the most loyal and essential members of the Kingdom of Science. He is a genius craftsman who can intuitively understand the properties of new materials and techniques almost instantly, often able to produce complex items from a simple diagram. This ability allows him to craft objects of such high precision that Senku himself admits Kaseki's skills surpass his own.

Kaseki's primary motivation is the pure joy of creation. The prospect of building something new, especially when presented with the scientific principles behind it, fills him with excitement and energy. Having felt isolated for much of his life due to his niche interests, he finds profound satisfaction and companionship in working alongside others who share his passion. His deep-seated desire is to be useful and to see his creations contribute to the advancement of the kingdom, and he often seeks validation for his hard work, particularly from women.

Within the story, Kaseki serves as the indispensable engineer and manufacturer for the Kingdom of Science. He is the one who translates Senku's theoretical knowledge and blueprints into tangible, working inventions. His role is crucial for almost every major project, from crafting basic laboratory equipment like glass flasks to forging sophisticated items such as Japanese swords, steam engines, water wheels, electrical generators, cameras, a hot air balloon, and the components for primitive cell phones. He also single-handedly oversaw the construction of the Perseus, a large sailing vessel. He is a key member of the crew on major expeditions.

Kaseki shares a particularly close bond with Chrome. They are described as kindred spirits due to their shared love for science, material gathering, and craftsmanship, and they frequently collaborate on projects. He also has a notable soft spot for Gen Asagiri, often pulling the mentalist aside to work with him on tasks, with Gen finding it difficult to refuse due to being intimidated by the old man's physical strength. He gets along well with Yuzuriha Ogawa, respecting her as a fellow artisan, and both share a deep-seated pride in their work that prevents them from doing anything sloppy. He also acts as a source of wisdom and encouragement for younger members like Ginro, teaching him that fear is a natural feeling that can be overcome for the sake of something precious.

Over the course of the series, Kaseki undergoes significant personal development. He transitions from a wary, isolated elder to a valued friend and a central pillar of the Kingdom of Science. He is deeply moved when Chrome refers to him, Senku, and Chrome as close friends, as he has not had such companions before. Despite his age, he is portrayed as being just as fired up and capable as the younger generation, and his physical vitality seems almost renewed by his involvement in scientific progress. He is a character who proves that passion and skill are not limited by age.

Kaseki's notable abilities are almost exclusively tied to his master craftsmanship. With over fifty years of experience, he can build almost anything by hand, from massive structures to incredibly delicate and precise components. He is a fast learner, capable of mastering blacksmithing or glassblowing with almost no instruction. His physical strength is also considerable, enough to tear through ropes and thick clothing, though he has little to no skill in direct combat. His intelligence is practical and spatial, allowing him to envision and construct complex mechanisms, making him one of the few people in the village with any form of technical knowledge before Senku's arrival.