Ensei Rou, also known as Ro Ensei, is a character introduced in the second series who serves as a former governor and a close ally to the protagonist, Kou Shuurei. He first appears as a half-starved traveler collapsed near Shuurei's residence, where she takes him in and offers him food, an act of kindness that begins their association. His initial appearance is deliberately unkempt, with messy hair and a shaggy beard that leads others to compare him to a bear, causing irritation for Shuurei and even allowing him to be mistaken for a kidnapper. Once he shaves this beard, he is revealed to be a handsome young man in his late twenties, noted for a distinctive cross-shaped scar located below his left eye.

Personality wise, Ensei presents a stark contrast to many of the more serious characters in Saiunkoku. He possesses a humorous, easygoing, and gentle nature, often serving as a source of comic relief. Despite his formidable strength and a violent past, he is consistently kind and caring toward the people around him. This gentle exterior, however, masks a tragic history; his entire clan was massacred by the Satsujinzoku, the Killer Gang of Sa Province, an event that shaped much of his early life and connects him to Shi Seiran, with whom he shares a complex and unscrupulous past. Ensei is a man who typically uses his body more than his brain, preferring action and physical solutions over deliberation, though he is far from unintelligent when the situation demands it.

Ensei was formerly the governor of Sa Province, a region notorious for corruption where officials were frequently assassinated or bribed. In an unprecedented move, the Taiho Sa Enjun declared that anyone who could defeat an assassin would be made provincial governor, leading to Ensei's appointment at the age of seventeen. Using his exceptional combat skills, he was able to competently foil assassination attempts and maintain order during his tenure. The death of his political backer, Sa Enjun, weakened his position as the legitimate governor, prompting Ensei to leave the province. His journey was motivated by a need to assess the new Emperor's political character, request the appointment of a replacement governor, and escape the high bounty placed on his head by the Sa family, which made him a constant target.

In the story, Ensei becomes closely tied to Shuurei's path. He acts as her temporary bodyguard, assigned by Seiran, and later becomes her assistant in the Censorate after her term as governor of Sa Province is suspended. His assistance is driven by a deep admiration for her dedication and ideals, and it is hinted that his feelings for her go beyond mere professional respect, though he intentionally conceals them and only a few, like Suou, recognize this affection. He also serves as temporary administrator for the Department of Treasury and Taxation under Kou Kijin, where he secretly defends against nightly thieves from Brown Province. Regarding notable abilities, Ensei is an excellent fighter renowned for his martial prowess with a large staff, earning him the nickname of the Little Staff King. However, his true specialty lies in close-range unarmed combat, a skill noted by Seiran, who acknowledges that his greatest strength is in hand-to-hand fighting despite his proficiency with a weapon. His character development shows a man who has balanced his traumatic, combat-heavy past with a new commitment to governance and justice, ultimately dedicating his strength to supporting Shuurei's mission.