Ensei Rou, also called Ro Ensei, collapses half-starved near Kou Shuurei's residence during a summer heat wave. Shuurei takes him in. His initial messy hair, shaggy beard, and rugged clothes lead others to compare him to a bear, irritating Shuurei and causing him to be mistaken for a kidnapper. He later shaves, revealing a younger, handsome face with a cross-shaped scar under his left eye, appearing in his late twenties.

His entire clan was massacred by the Satsujinzoku (Killer Gang) in Sa Province. During this time, he encountered Shi Seiran, then part of the same gang. Ensei earned the name "Little Staff King" for his skill with a large staff, though his true specialty is close-range unarmed fighting. Seiran notes his formidable strength despite his gentle manner. This shared past creates a complex, often tense dynamic between them.

Unexpectedly at seventeen, due to corruption and assassinations in Sa Province, the Taiho Sa Enjun declared anyone thwarting an assassin would become governor. Ensei succeeded, becoming temporary governor. He maintained order and stopped assassinations using his combat skills. After Sa Enjun's death eroded his political backing, he voluntarily left Sa Province. His departure was partly to evaluate the new Emperor and request a replacement governor, but also to escape a bounty placed on him by the Sa family, making him a target during his travels.

He later becomes Shuurei's temporary bodyguard, assigned by Seiran. Subsequently, Kou Kijin appoints him temporary administrator for the Department of Treasury and Taxation, where he secretly defends against nightly thieves from Brown Province. After Shuurei's term as Sa Province governor ends, he assists her as an assistant in the Censorate, driven by deep admiration for her dedication and ideals. His feelings for her extend beyond professional respect, though he conceals them; only Suou recognizes this affection.

Personality-wise, Ensei contrasts serious-minded characters with his humorous, easygoing demeanor. Despite his imposing strength and violent past, he shows consistent gentleness and kindness. He prioritizes action over deliberation, embodying his philosophy: "It’s better to spend your life regretting doing things than to spend it regretting not doing things." His development balances combat-honed instincts with a commitment to governance and justice, shaped by his trauma and dedication to Shuurei's mission.