In Natsume's Book of Friends, some of the human-looking Youkai wear masks, or at least obscure their sight. For example:
Tsubame
Hotaru
Hiiragi
Tsuyukami
Sasago
I am wondering if there is a reason for this as none of these Youkai seemed to be affected by their obscured vision. For Tsubame and Hotaru, they are both shown to be able to show their full face. And for Kiiragi, after she was attacked, we can see one of her eyes though her broken mask and there seemed to be nothing wrong with it.
From this article on Japanese masks and their meanings:
In ancient times, masks were used to portray creatures and characters during plays and rituals.
It seems that that carried over into manga and anime to represent youkai when in human form, much like the common depiction of a ghost wearing a triangle-shaped bandage on the head.
Noh plays in particular seem to be a common style source for masks worn by human-form youkai characters. Many of the fox, cat and other masks shown at festivals derive from Noh mask style and have been used to represent their respective creatures.
I've seen manga and anime that uses masks depicted in the more traditional style as portrayed in that article (particularly the Hannya and Oni masks), masks that are styled after Noh masks, and simple cloth coverings over the face.
I expect that in Natsume's Book of Friends, it's simply a stylistic choice by Yuki Midorikaw more than anything else to depict youkai in that manner.
