Yama kei is a mountain based natural style, often considered a sister style, or substyle, to mori girl.
The Fashion Edit
Yama kei differs from mori primarily in its use of layers, color schemes, and use of athletic gear.
Generally, yama kei is made up of simpler layers than mori girl. For instance, a shirt with a cardigan paired with a pair of pants could be considered yama. Sometimes outfits will have more layers, but it is certainly not required. Additionally, pants are much more widely used in yama than mori.
Color schemes in yama kei can be natural colors, similar to mori, but may also include neons and bright colors as well, as these colors are often seen on sportswear.
Lastly, yama uses many aspects of sportswear, such as vests and hiking boots, that are unique from mori kei.
Yama kei, like mori kei, has a natural lifestyle attached. However, yama kei's lifestyle is based on its name (yama means mountain in Japanese), and is focused on hiking, camping, and other "mountain-like" activities.
Yama kei had brief popularity with magazines, brands, and even in anime with "Yama no Susume" featuring various yama girls and a mori girl. However, the style quickly fell out of popularity, and never caught on quite like mori kei, leaving it now inactive.