Mamie is Kun-chan's great-great-aunt, a prominent ancestor in his family lineage. She lived during the early 20th century as the daughter of a wealthy family, raised in luxury on a grand estate. This privileged upbringing granted her education and refinement, yet she notably defied the restrictive gender norms of her era by developing a passion for motorcycle riding, an unconventional pursuit for young women at the time.

Her character arc centers on a pivotal romantic relationship. She fell in love with a man her family deemed unsuitable and chose to elope with him. This act of defiance led to her immediate disinheritance, resulting in the permanent loss of her family fortune and social standing. The experience profoundly shaped her worldview, instilling a deep sense of caution regarding impulsive actions and their long-term consequences.

Mamie interacts with Kun-chan directly through the film's time-travel narrative device, appearing as a young woman from her past. In these encounters, her primary role is to impart lessons about responsibility and the consequences of choices. She explicitly warns Kun-chan against reckless behavior, using the loss of her own wealth and status as a concrete example of how impulsive decisions can lead to significant hardship. Her narrative functions as a cautionary tale within Kun-chan's journey to understand his family's history and his place within it.

Her background story remains consistent across her appearances, focusing exclusively on her youth, her rebellion through elopement, and the resulting financial and social downfall.