Sana's younger sister, who is also Keiko's daughter. She makes appearances in four pages of the story and is featured on the cover of the final book. Mariko is a big fan of Sana, unaware that Sana is her older sister. After meeting Keiko, Sana spends a day having fun at a theme park with Akito, Tsuyoshi, and Mariko. Despite enjoying her time with Mariko, Sana realizes she doesn't feel a familial connection with her or Keiko.
Episode 023 of the anime focuses on Mariko. In this episode, she wanders off to see Sana after a remote for Child's Toy is shot near her house. She then boards the channel 108 bus, unknown to everyone except Zenjiro, and roams around the studio. Keiko ends up calling Misako for help in finding her "lost" daughter.










