Miki is a young girl who lives in the slums of Shinjuku and later becomes the adopted daughter of Umibozu. She attends second grade at Shinjuku Elementary School. Known as the Little Girl of Fortune or Lucky Little Ghost, she gains a reputation as an urban legend among those who encounter her. People who meet her are said to be charmed by her smile and often experience good fortune afterward.

Her background is marked by hardship and loss. After her mother passed away several years before the events of the story, Miki lived alone in an underground section of Shinjuku Station. Her mother had raised her in secrecy, hiding in an abandoned underground arcade to protect them from potential danger. Miki never knew her father, though it is later revealed to other characters that he was the King of an unnamed country. He had fallen in love with Miki's mother while visiting Japan on business, but due to an unstable political situation in his kingdom, she left her job and disappeared when she became pregnant. After her mother died in an accident, Miki was discovered by Saeko, and later, following a hit-and-run accident, she was taken in and adopted by Umibozu. She subsequently moves into Cat's Eye Cafe, where she lives with him.

In terms of personality, Miki is depicted as a warm and gentle child. She has a strong sense of justice and dislikes violence. Despite her young age and the difficult circumstances of her past, she maintains an optimistic and kind demeanor that draws people to her. Her motivations are not complex; she seeks connection and stability, finding both in her new family at the cafe.

Her role in the story is primarily that of a supporting character who brings a sense of innocence and hope to the darker elements of the narrative. She serves as a reminder of the possibility of new beginnings and the importance of protecting the vulnerable. Key relationships include her bond with Umibozu, who becomes her father figure, as well as interactions with Ryo Saeba and Xiang-Ying. Later in the series, Ryo and Xiang-Ying discover that the King has been killed and replaced with a lookalike, but these events are kept from Miki to shield her from harm.

Over the course of the story, Miki undergoes a subtle but meaningful development. She transitions from a solitary orphan surviving in the shadows of the city to a cared-for child with a stable home and a loving adoptive parent. Her presence also affects those around her, as her innocence and optimism help soften the hardened lives of the other characters.

Miki does not possess superhuman abilities, but she is credited with a kind of luck or good fortune that seems to follow those who meet her. This quality, combined with her friendly and trusting nature, makes her a beloved figure among the characters who know her.