Sprite

Yoshiko is a hardworking high-school student whose free time is almost entirely taken up by attempts to help out a hikikomori childhood friend and an odd, reclusive uncle.

While on a visit to her uncle's penthouse with two of her friends, Yoshiko notices a black substance that resembles snow beginning to fall from the sky -- but her friends see nothing. Soon afterwards, a massive earthquake shakes the city, and Yoshiko, her friends, her uncle and his dog, all find themselves trapped in a strange world they do not recognize...with an assortment of weird people to keep them company.

Note: Volumes 6 and 7 contain a four-chapter short manga, also named Sprite. It acts as a prequel and tells the story of Fuyuko and Shingo.