Emiko is a central supporting character known for her passionate, bold, and relentlessly energetic personality. She is the wife of Kosuke Niwa and the mother of the protagonist, Daisuke Niwa. Emiko does not act her age; she possesses a hyperactive and bubbly demeanor that often distracts from her shrewd and masterful skills in deception and disguise. Despite her sometimes comically inconsiderate treatment of her son, she is deeply protective of her family and is quick to suspect or threaten violence against anyone who harms them.

Her background is rooted in the legacy of the Niwa family. As the daughter of Daiki Niwa, who was the tamer of the phantom thief Dark a generation before, Emiko grew up knowing the family secret. However, she was deeply upset to learn that because she was female, she could not inherit the family ability to transform into Dark Mousy. This realization left her feeling guilty and sorrowful, believing she had taken away her father's most precious person. Unable to become the phantom thief herself, her dream evolved into becoming the mother of the phantom thief. As a young woman, she famously rejected her many suitors by questioning whether they would be suitable to father her future son, a habit that scared most of them away. She eventually met and married Kosuke Niwa, who was the first person to ask her why she wanted a son so badly, rather than simply rejecting her. She explained her desire to be Dark's mother, a dream she achieved with the birth of Daisuke.

Emiko’s primary motivation is to fulfill the Niwa family duties, a role she takes to heart with immense enthusiasm. Since Daisuke was an infant, she has worked alongside her father to rig their home with deadly traps and elaborate training courses to prepare her son for his life as Dark Mousy. Although she acknowledges she should have explained the reasons for his actions sooner, she takes great pride in her son's transformation, finding it "cute." Her role in the story is that of the mastermind behind the heists: she is responsible for designing and sewing Dark's flashy costumes, and she writes and sends the detailed warning notices to the police, specifying the target, date, and location.

Her key relationships define her actions. She is very much in love with her husband Kosuke, a shy and thoughtful man who married into the Niwa family. With her son Daisuke, she plays the role of a demanding and eccentric trainer, viewing his struggles with a surface-level nonchalance that belies her inner care and concern for his well-being. She also sees Dark as a son, a sentiment that stems from her childhood dream, and she takes her responsibilities as his "mother" very seriously.

Throughout the story, Emiko shows limited development, as her core personality remains consistently vibrant and unchanging. However, her past is explored in depth, revealing the sadness behind her seemingly frivolous quest for a son. A significant moment occurs on her birthday, when Dark and Daisuke work together to thank her for her wish to be his mother, a gesture that fulfills her long-held emotional need for recognition.

Emiko possesses several notable abilities beyond her domestic skills. She is a master of disguise and sleight of hand, skills she uses to support Dark's activities and to distract observers. She also has a creative talent for crafting, producing both the intricate traps in the Niwa home and the distinctive costumes worn by Dark. In the anime adaptation, her hyperactivity is heightened, and her role in making the thief costumes is more prominently featured.