You see, I make copyright-free characters. I've been using the tag Asset, but I'm sure that's not accurate. Would it be possible to create a public-domain/copyright-free tag?

Royalty Free: Royalty-free images are digital assets that can be used multiple times without paying additional fees for each use. This licensing model offers flexibility and cost-effectiveness for content creators and businesses seeking high-quality visuals. (oliverpiro.com).

Copyright: the exclusive and assignable legal right, given to the originator for a fixed number of years, to print, publish, perform, film, or record literary, artistic, or musical material (Bing)

Assets is the correct tag.  I think there isa tag for free. You don't want to give away the copyright on your assets, because then anyone can take them and resell them as their own creations. Instead, you should make them royalty free so developers can freely use them in their games. 

Licenses are already implemented, but they're set on the Metadata->Release Info tab on your project dashboard, not via tags. You can pick Creative Commons Zero, which is as close to public domain as it gets, but are you sure this is what you want? See our friend's advice above.