Omasa is a supporting character in the anime Onihei, a female spy who works for Heizō Hasegawa. She is an adult woman with black hair usually worn up and brown eyes. Her history connects her to Heizō's youth: she was the daughter of Chūsuke, the owner of a tavern in Honjo that the young Heizō frequented when he was still known as Honjo no Tetsu. At that time she was a girl of about ten or eleven and already felt affection for him, often looking after him, such as preparing egg sake when he was drunk. The two lost contact for more than twenty years.

By the time she reappears, her life had taken a grim turn. After her father retired from a life of crime, he had placed her under the care of a thief named Jinzaburō of Yoshima, and she became a hikikomi-onna, a decoy who lured victims into the hands of burglars. She tolerated this work out of gratitude and because the robberies she aided had not involved harming anyone. When Jinzaburō planned a robbery that required killing the household, she was repelled by the cruelty and resolved to break away. She went to Heizō, revealed the plot to him, and volunteered to become a spy, risking her life in his service. Heizō accepted her, rescuing her from her former accomplices, and from that point she worked as a spy within his information network.

As a spy, Omasa's usual cover is that of a cosmetics peddler, using white-and-red face powder and small wares to blend into the streets of Edo while conducting surveillance and trailing suspects. Having once lived as a free thief and endured many dangerous situations, she carries an exceptional memory for the faces and names of gang leaders, underlings, and decoy women across the underworld. This makes her a valuable source of intelligence for Heizō's office, as she can identify criminals and connect people who might otherwise seem unrelated. She operates among other informants, most notably alongside the veteran spy Hikojū.

In personality, Omasa is reserved and rarely shows emotion openly, which suits her work in the shadows. She is brave and willing to place herself in danger, but she is also mindful of the social distance between herself and Heizō; despite her devotion, she does not presume to enter his household as an equal and keeps her attachment to him unspoken. Her compassion becomes most visible when she encounters women in circumstances similar to her own past, as their suffering stirs her feelings more than her own hardships do. She values the human dignity and the sense of honor that Heizō taught her, and loyalty to him is the strongest force in her life.

Her relationship with Heizō defines her character. She loves him, but the difference in their social standing makes a romantic connection impossible, so her affection expresses itself as devoted service and fierce loyalty. Heizō, for his part, trusts her with dangerous work and shows her a respect she had never experienced before. The other significant relationships in her life are ties to the criminal world she left behind: her former master Jinzaburō, whom she ultimately betrays by siding with justice, and former comrades who sometimes try to draw her back into that life.

Across the longer span of her story, Omasa undergoes considerable development. She moves from being a tavern girl and a reluctant tool of thieves to a capable and trusted operative. At times she is pulled back toward her old associates, and in some of her later stories she even falters and turns against Heizō before being won back by his compassion. Eventually she finds a degree of stability, stepping back from constant danger and settling down as the wife of a man named Gorozō, though her years as a spy remain an essential part of who she is.