Сакай Маріко

Sana's little sister, who is Keiko's daughter. She appears in a total of four pages, but also on the cover of the final book. She is a fan of Sana, not knowing that Sana is her older sister. After meeting Keiko, Sana spends one day having fun at a theme park with Akito, Tsuyoshi and Mariko. Although Sana enjoys her day with Mariko, she realizes that she doesn't feel a familial bond with her or Keiko. Episode 023 of the anime is dedicated to Mariko, where she wanders off to see Sana when a remote for Child's Toy is shot right by her house. She then gets on the channel 108 bus, unbeknownst to everyone but Zenjiro, and then wanders around the studio.

Keiko ends up calling Misako for help in finding her "lost" daughter. From: Wikipedia.org