Sorincha

ソリンチャ

In the Japanese version, Hikora refers to his sister as "Imoto," the Japanese term for "younger sister." Consequently, it is inferred that Sorincha is Hikora's younger sister.
Sorincha went to play O-to-ko-yo and did not return. She appears to be possessed by a demon, as her mask transforms into that of a demon at the end of the film—specifically, the "Fifth Demon" in the city. The possession occurred because she was the last child remaining and had not been caught the night before, resulting in the possessed mask being placed on her and making her "it" for the next game.
It remains unknown what happened to Sorincha at the conclusion of the game, as she is not shown at the end.