I stumbled upon this hitaikakushi tag a while ago and found out that it's terribly underused, considering how many saigyouji_yuyuko posts there are. After I decided to mass-tag Yuyuko posts with this for several pages, I then thought of how many people are actually going to search for this tag; evidently from the number of posts with hitaikakushi, not nearly enough. This looks to be a rather uncommon japanese word on Danbooru.
Not sure if Yuyuko's thing applies, since it has a Dreamcast spiral on it, not completely blank.
That triangular thing certainly is something to be tagged, but with another name more descriptive for non-Japanese speakers, which is nearly all of Danbooru. What I have in mind would be something like ghost_headpiece or triangular_headpiece; suggestions welcomed, of course. hitaikakushi would then be aliased to the new tag.
Unrelated: I always typed Danbooru as Danboory before correcting the mistake.
Updated by jxh2154