Shikigami

Would it worth implicating characters who are Shikigami, such as Chen or Yakumo_Ran, as such?

No.

Alright

Kinda agree with RaisingK here. I mean do people actually search for things like Shikigami when they come to Danbooru? I'm not sure most people even know what's shikigami is or means upon first glance so it possibly won't be a useful tag.

shikigami would probably make a more useful tag for the paper ones like ichijou akari uses. (though only post #296604 seems to contain them)

It's generally our policy never to implicate character tags to anything.

7HS said:
shikigami would probably make a more useful tag for the paper ones like ichijou akari uses. (though only post #296604 seems to contain them)

This is probably for the best, as it's the only way I can think of that is visually distinct enough for such.
Unless there's already a tag for those paper-doll familiars, in which case, it's probably fine.

The thing is, being a "shikigami" (aside from the above-mentioned type) isn't exactly visually distinctive, so any tagging like that would be using extraneous information. Generally speaking, tags apply to the visual elements present in an image only, with only things like seiyuu jokes, neta, and the series from which the characters in the image originate being the exception.