Mori Kei (森系) or Mori Girl (森ガール) is a Japanese fashion style that centers around an appearance that is inspired by living in the woods.  It's basic aspects is having an appearance that combines clothing that appears natural, girly, layered, and loose.  The loose layers typically hide the body shape and give what is described as a "fluffy appearance" (ゆるふわ).
mori kei