From MangaHelpers:

A tsukumo-gami is an artifact spirit, which comes into being for very old, and cherished objects. One day, a tsukumo-gami--later named Kyou--is born from an old mirror. Although she remains close to her mirror, she has the physical, movable body of a tiny, dark-haired girl. As she explores the world around her, she makes her first friend, a mouse!

Soon her mirror is bought by a girl who can actually see and speak with her, though she professes to have no particular interest in tsukumo-gami. In that way, Kyou and her mouse friend come to their new home, which is a store. Soon she finds another friend in the worker Akane, a girl who can't see her, but who finds her fascinating nonetheless. As Kyou's horizons expand, thanks in a large part to her discovering that she can travel through mirrors, she meets other new spirits and people! [tethysdust]