Kaijin is a dwarf blacksmith and former commanding officer in the Dwarven Kingdom's military. Despite his commoner origins, he held semi-baron status. He led a faction within the Dwarven Engineering Corps advocating for steady technological advancement, opposing his rival Vesta's push for rapid innovation. When Vesta's failed Magisoldier project caused disaster, Vesta falsified evidence to blame Kaijin. This led to Kaijin's unjust expulsion from the military and exile by King Gazel Dwargo. His exile was compounded after he assaulted Vesta for insulting Rimuru Tempest, a monster who had saved dwarf miners.

Following his exile, Kaijin joined Rimuru Tempest in the Jura Forest, accompanied by his childhood friends and fellow craftsmen Garm, Dold, and Mild. He played a pivotal role in transforming Rimuru's nascent settlement into the Jura-Tempest Federation. Initially focused on forging weapons and managing metallurgy, his technical expertise and leadership in production coordination were critical in establishing Tempest's foundational infrastructure, elevating it from a village to a functional civilization. Over time, he transitioned from direct forging to overseeing broader technological development, eventually delegating weapon production to Kurobe while specializing in research and production management.

Kaijin later reconciled with Vesta, collaborating on advanced projects like the Spirit Magic Core. This partnership contributed to Tempest's technological innovation under Rimuru's rule. Kaijin's responsibilities expanded to include city planning and construction, exemplified by his deployment to Milim Nava's territory to assist in building a new city.

Physically, Kaijin fits typical dwarf characteristics with a short, muscular build, brown skin, a prominent beard, and blue eyes. He wears a pointed earth-toned hat with goggles, a fur-trimmed purple shirt, and sturdy boots. Personality-wise, he balances a serious work ethic with occasional mischief, such as proposing a mixed-gender bath for voyeuristic purposes – an idea swiftly vetoed by female colleagues. He demonstrates fierce loyalty, defending comrades against slights and honoring commitments rigorously, as seen when fulfilling Rimuru's request for village construction after Rimuru aided him with a critical sword order. His leadership style emphasizes teamwork and inclusivity, extending opportunities even to former rivals like Vesta.