Sub-character tag feature

I was wondering on the need of tagging characters in artworks where some characters only appear sketched out in a corner/in the background, when suddenly an idea beamed into my mind:
"What if there were sub-character tags?"
By that i mean - we already have a sub-division of copyright, artist, character, general, and lastly meta tags. Wouldn't it be possible to add another one on a case-by-case post basis? For example, let's take our dear eudetenis, post #3505027. This image has a LOT of character and copyright tags, but it is only focused on one character. By dividing the character tags into "main" and "sub", people who don't know the character will instantly be able to know who it is without checking out other unrelated character tags. Another use would be so artworks like this don't pop up on searches of a certain character, unless you add the tag to your search.
Another use would be for characters almost completely out of frame/appearing in the background, so for example Seiga in post #3566942.
Lastly, i guess it could be used for the increasing amounts of Fate and Azur Lane alternate character tags.

Related: topic #15982

That sounds like a lot of work for little to no payoff. Currently, filtering through search results requires minimal effort. It's usually apparent from just the thumbnail alone which characters are being focused on in an image.

To make this work, we would basically have to create new tags in this category for every character that has ever appeared like this in an image, which would be a LOT of work.

It would be preferable if we could qualify within a post itself which characters were "in focus" and which were not, without the need for more general, site-wide searchable tags. But danbooru doesn't currently have that kind of functionality.

Kayako said:

It would be preferable if we could qualify within a post itself which characters were "in focus" and which were not,[...]

That's pretty much the idea i had in mind