Kaijin was a blacksmith of the Dwarven Kingdom, and the brother of Kaidou. After he was exiled from his own country, he became Rimuru Tempest's subordinate, and helped build the Monster City "Tempest," he acted as a builder, blacksmith, and craftsmen. He also was the former commander of the Dwargon Engineering Corps.
He might look like a big softy, but he's actually an extremely prudent old man.
Thanks to having made it up to the position of a commanding officer, he was granted the status of a semi-baron.
Without having produced a single technological innovation, the dwarven engineer corps were resigned to being the least evaluated corps. The engineer corps was split into two factions, Vesta's and Kaijin's. Vesta's faction insisted that considering that our country is the founding nation of technology, the engineer corps should be the star of the knight order. Kaijin's faction had refuted, instead advocating that they should steadily advance their research, as they had done up till then. The arguments of both sides only resulted in a clashing of opinions, and their meetings never ended in a conclusion.
And it was during that period of struggles, that one elf engineer had initiated a joint development plan that was named, the “artificial magic soldier project.” Vesta had wanted to make that project succeed no matter the cost, all in order to redeem the honor of the engineer corps, once and for all. Although Kaijin had pointed out that Vesta was being too impatient, he had paid no heed at all, because the warning was given by a superior of commoner's descent. At that time, only the best of engineers had assembled to participate in the project. But as a result of his over-eagerness, Vesta had ended up causing the rampage of the “enchanted soulstone,” and the project was brought to a standstill. And it was in this way, that the “artificial magic soldier project” met its demise.
At the end of the day, Kaijin was the one to take full responsibility for that failure and forced to leave the army. This was because Vesta, on top of pushing all blame onto Kaijin, had even bribed the upper echelons of the army to prepare false evidence.
Kaijin and his company were reluctantly banished from Dwargon by Dwarf King Gazel. That happened after his altercation with Vesta, who had insulted the newcomer slime Rimuru, an honored guest and savior of his compatriots. In truth, however, he wanted to work under Rimuru as his expert blacksmith in the growing nation of Tempest, rather than have to face Vesta again. It was thanks to him and the other dwarves with him that Tempest became a proper civilization.
For Vesta who was nobility, it was humiliating for him to be forced to take orders from a commoner.