Koharu is introduced as the younger sister of Roman, the phantom thief at the heart of the story. She lives a double life of her own, running a decorative chopstick shop by day and serving as a capable assistant to her brother during his nighttime missions to reclaim stolen property. Originally believed to be Roman's blood relative, she is later revealed to be the last princess of the Tokugawa clan, the daughter of the thirteenth shogun, Iesada Tokugawa. Her real name is Haru. She was entrusted into the care of Jirokichi by her mother, a traveling performer who died in childbirth, in order to protect her from the scandal that her parentage would cause.

Koharu is portrayed as responsible, perceptive, and compassionate. She approaches her work and her role in Roman's activities with a sense of duty and a sharp eye for detail. Though she is not a combatant, she provides vital tactical and logistical support, often coordinating from the background or keeping watch. Her loyalty to her brother and her belief in his cause drive her to take risks, even when it puts her in danger. She also shows a caring side toward those around her, including their dog, Sakura.

Her role in the narrative shifts significantly when she is kidnapped by the antagonist, Admiral Perry. This event becomes a personal crisis for Roman and a turning point in the series. Perry subjects Koharu to brainwashing and forces her to operate a massive cannon, which she fires on Edo, causing heavy destruction. Her captivity and eventual rescue form the core of the story's climax. Through this ordeal, she evolves from a supportive family figure into a central pawn in a larger geopolitical conflict, and her recovery underscores the final victory.

Koharu's most important relationship is with Roman, whom she trusts and admires. Their bond remains strong even after the truth about her lineage comes to light. She also shares a connection with Jirokichi, who raised her in secret. Her development touches on themes of identity, loyalty, and resilience, as she confronts her true origins and overcomes the manipulation imposed upon her. She ultimately emerges as a more self-aware character, still defined by her kindness and her commitment to those she loves.