Mariko Niki, formerly Mariko Futaki, works at the Tozai News newspaper in the Culture Department alongside Shirō Yamaoka and Yūko Kurita. She comes from the affluent Futaki family, which had no male heir. Following tradition, her family arranged for her to marry Yamaoka so he could be adopted as the family's male successor, presenting substantial financial and social incentives.

Early in the series, Mariko actively seeks a romantic relationship with Yamaoka. This storyline reaches its conclusion in the chapter "The Spirit of the Sardine," where Yamaoka definitively rejects her marriage proposal. Her aunt, Teruko, conveys the family's expectations concerning this arranged marriage.

Following Yamaoka's rejection, Mariko's story shifts focus primarily to her professional duties at the newspaper. She does not appear in the plot of the special *Oishinbo: Kyūkyoku Tai Shikō, Chōju Ryōri Taiketsu!!*, which centers on Yamaoka and Kurita's culinary research in Okinawa for a longevity cooking competition against Kaibara Yūzan. Information about her portrayal in other adaptations beyond the manga is unavailable.