Solo mission

As a part of her grade, Ruby has to take a week long solo mission. But when her mission is miscalculated on how many more grim infested the emerald forest. She is overwhelmed and faints from exhaustion but when she wakes up is found by Kei and discovers that she's in a strange city that is more rudimentary to Remnant. What will happen? why and How is she in this Tokyo? Will she ever get home? and what is a Ajin?

Disclaimer I don't own the picture