Mariko Sakai is the younger half-sister of Sana (though she does not know it) and the daughter of Keiko Sakai. She is a large fan of Sana, despite not knowing of their blood ties, and would enjoy a day at an amusement park with her when Sana met her and her birth mother.

After having met with her sister, Sana had intended on not seeing her again, saying she didn't feel a familial bond. Mariko one day wanders off from her parents in search of her when hearing she is shooting nearby. She doesn't get to meet her again, but does spend much of the day with Zenjiro, whom she annoys greatly. Despite her being a pest to him, he searches for her parents in his car, who upon finding them, attack him thinking he is her kidnapper. When the misconception is cleared, Zenjiro is relived to give her away, but is less mean to her.