Kaijin, a dwarf from the Dwarven Kingdom, rose to the rank of semi-baron despite his commoner origins. He commanded the Palace Knights' Workshop Team and led a faction within the Dwarven Engineering Corps. His rivalry with fellow researcher Vesta escalated during the failed Magic Soldier project. When Vesta's reckless actions caused the project's collapse, he falsified evidence to blame Kaijin, resulting in Kaijin's unjust exile alongside childhood companions Garm, Dold, and Mildo. This exile followed an altercation where Kaijin defended Rimuru Tempest against Vesta's insults.

Exiled to the Jura Forest, Kaijin dedicated himself to Rimuru Tempest's emerging nation. His expertise in blacksmithing, weapon forging, and project management proved vital for building the Jura-Tempest Federation's technological and infrastructural foundation. He personally oversaw weapon production and construction, delegating specific roles: Garm managed textiles and clothing, Dold handled tools, and Mildo specialized in architecture and urban planning. Their combined efforts rapidly advanced the federation. Kaijin later shifted primary weapon-making duties to Kurobe to focus on broader research and production management.

Kaijin's relationship with Vesta transformed significantly. After Vesta's own banishment and arrival in Tempest, Kaijin advocated for his acceptance despite their past. This led to reconciliation and a productive research partnership under Rimuru's leadership. Together, they pioneered advancements like the Spirit Magic Core. Kaijin's collaborative spirit extended to assisting in building a new city within Demon Lord Milim Nava's territory, showcasing his trusted role beyond Tempest.

Physically, Kaijin displays typical dwarf traits: short, muscular build, brown skin, and a prominent bushy beard. He typically wears a pointed earth-toned hat with goggles, a fur-trimmed purple shirt, a waist cloth, and sturdy brown boots. His piercing blue eyes radiate determination.

Personality-wise, Kaijin balances diligence and wisdom with fierce loyalty towards comrades. He takes deep pride in his craftsmanship and rigorously honors commitments, exemplified by fulfilling his pact with Rimuru to forge ten magical swords under a strict deadline. Generally prudent, he possesses a mischievous streak, seen in his ill-fated proposal for a voyeuristic mixed-gender bath, swiftly vetoed. He also enjoys leisure, such as visiting elven hostess bars. His protective nature emerges when allies are threatened, as when confronting Vesta over insults against Rimuru. Despite enduring betrayal and false accusations leading to exile, he harbored no lasting resentment.

Within the slice-of-life context of "The Slime Diaries," Kaijin features in Tempest's daily life and minor events, reflecting his integration into the federation's community.