Sei Kai held the rank of Great General in the Han military, commanding the Sei Kai Army during the Coalition Invasion. Originally noted for remarkable beauty that attracted even men, prolonged exposure to poisons during extensive toxic research transformed his appearance: his skin became gray and marked by prominent black veins, his eyes bloodshot with dark sclera, and his overall complexion paled. This research involved experimenting with venomous flora, fauna, and minerals gathered from distant regions, causing over a hundred fatalities among his men and leaving survivors with physical impairments. He led 50,000 troops specializing in unorthodox warfare centered on poison-based weaponry. His forces included the Black Arrow Unit, equipped with poison arrows and crossbows, and utilized projectile weapons like catapults firing poison gas spheres. Sei Kai developed two primary poisons: one inducing rapid death upon wounding, deployed by archers, and another acting slowly over eight days, delivered via ballista-launched gas containers. At the Battle of Kankoku Pass, Sei Kai initially remained passive. On the seventh day, he deployed his poison corps, targeting Qin commanders atop the fortress walls. This tactic succeeded in poisoning the elderly Qin general Chou Tou with a projectile called Tangan. On the fifteenth day, during a coalition full assault, the critically weakened Chou Tou participated in a covert operation led by Kan Ki. Disguised as Wei soldiers, their small force infiltrated the Han headquarters. Despite defense efforts by Sei Kai's subordinate Na Kon and the Black Arrow Unit, Chou Tou breached the command center and beheaded Sei Kai. Chou Tou then succumbed to the poison, resulting in both leaders' deaths. Sei Kai consistently displayed a calm and reserved demeanor, rarely speaking during strategic meetings and showing minimal emotional reaction to battlefield events. His approach to warfare was pragmatic and detached from traditional honor, viewing poison as an efficient killing tool. He demonstrated fearlessness by handling lethal toxins directly and maintained effective command over large-scale operations. His tactical acumen emphasized exploiting enemy vulnerabilities through ranged attacks combined with poison applications. This leadership earned him the rank of Great General despite limited battlefield experience, officially noted with a low "C" rank in that category. His physical combat abilities were minimal, relying on his elite guard for close protection. Following his death, command of the surviving Han forces transferred to Chou In. The Sei Kai Army disbanded after the battle.