Sarutobi Sasuke is a foundling with unconventional origins, rescued from a crashed spaceship and raised by the Sanada clan. This origin shifts from folklore's monkey-reared orphan yet preserves his core identity as an outsider integrated into the Sanada forces. Serving as a ninja under Sanada Masayuki and his son Nobushige during the Keicho era, his primary duty involves defending Ueda Castle against Tokugawa Hidetada's army, which aims to seize the castle to solidify Tokugawa control over Japan.

He employs his ninja skills in sabotage, psychological warfare, and tactical operations against the numerically superior Tokugawa forces. These actions resonate with historical legends of the Sanada Ten Braves, where he features as a key member utilizing guerrilla tactics. His exceptional agility and strategic acumen are applied to disrupt supply lines, gather intelligence, and execute surprise attacks, delaying Hidetada's advance toward the Battle of Sekigahara. The narrative frames his actions within the Sanada clan's broader struggle, emphasizing outmaneuvering the Tokugawa through wit and unconventional methods over direct confrontation.

His character development centers on loyalty to the Sanada cause and his adaptation from foundling to dedicated warrior. Interactions within the clan highlight themes of allegiance and identity. The story consistently positions him within the ensemble of Sanada retainers, focusing on their collective resistance.