Mariko "Grandma Marie" Juumonji is the sister of Sayuri Tamura (Sally) and the grandmother of Kazuma Tamura. As the sister-in-law of Tomio "Tommy" Tamura, she played a pivotal role in Tommy and Sayuri's early relationship. Mariko deliberately misled Tommy, falsely informing him that Sayuri did not like him, intending to create distance between them. Ironically, this deception had the opposite effect, ultimately facilitating Tommy and Sayuri's marriage.

Within the series, Mariko appears solely in flashbacks and old photographs, as she is deceased during the main timeline. Her actions form a significant part of the Tamura family's backstory, specifically shaping the circumstances that led to Tommy and Sayuri's union.