Kaseki is the elderly but exceptionally skilled artisan of Ishigami Village, and a descendant of the astronauts who originally survived the石化 event that petrified humanity. His name itself is a play on the Japanese word for fossil, a fitting reference to his advanced age relative to the rest of the cast. Despite being a senior, standing at 155 centimeters tall, Kaseki possesses a deceptively powerful physique that he reveals when fully engaged in his craft, often bursting through his clothes with sudden muscle growth.

In terms of personality, Kaseki is an honest and prideful old man who initially lived a relatively isolated life, as his fellow villagers viewed his passion for creating things with disdain. To cope, he focused on practical projects like building the village bridges and crafting shields, such as the one later used by Kohaku. He is unassuming when idle but displays boundless energy and enthusiasm when excited about a new challenge. Kaseki seeks validation for his work, particularly from women. Although wise and unafraid to admit his shortcomings, he is also a perfectionist who cannot tolerate sloppy work, a trait he shares with fellow artisan Yuzuriha.

Kaseki is primarily motivated by his lifelong love of crafting. He spent fifty years honing his skills in a primitive environment, and Senku Ishigami offers him the opportunity to create things he never dreamed possible. While initially wary of Senku, Kaseki quickly became his most loyal supporter. He finds Senku's scientific theories amazing and eagerly transforms abstract blueprints into physical reality, from glass vessels and vacuum tubes to engines and entire ships. Kaseki respects Senku as his chief and considers him a close friend, a sentiment he also shares with Chrome, the village sorcerer. Alongside Chrome, Kaseki is one of the original two villagers with pre-existing scientific curiosity. They bond over their shared interests in gathering materials and building.

Kaseki serves as the irreplaceable technical backbone of the Kingdom of Science. He is the one who physically creates the complex instruments necessary for Senku's inventions, allowing the group to advance technologically at an unprecedented rate. His role is not merely that of a laborer; his intuition with materials is so sharp that Senku admits Kaseki's skills far surpass his own. He can pick up techniques with little effort, learning to blow glass simply by watching Senku's failed attempts and forging multiple katanas in a fraction of the time it would normally take. Beyond his work, he often imparts wisdom and has a notable soft spot for Gen Asagiri, frequently pulling the mentalist aside to assist him because Gen is intimidated by Kaseki’s physical strength.

Throughout the story, Kaseki is central to nearly every major construction project. He helps build the water wheel, the cellphone, the hot air balloon, and the Perseus, a massive ocean-going ship. Despite his physical resilience, he suffers from chronic back pain due to his age. Interestingly, the petrification and revival process temporarily heals his body, rejuvenating him and providing a brief respite from his aches before he returns to work. During the Treasure Island arc, he is petrified and his body is shattered, only to be revived again to help Senku construct a drone.

Kaseki is a master artisan with over fifty years of experience. His notable abilities include an almost supernatural understanding of material properties, allowing him to work with substances like glass with no prior exposure. He constructs the intricate internal components for inventions such as cellphones, generators, and cameras. Physically, he demonstrates immense strength for his age, tearing through ropes and wielding tools for hours on end without fatigue. His craftsmanship is so legendary that his image was even printed on a new currency created by the Kingdom of Science.