This is just another case of related tags pollution. What happens is that when a new character comes out, people don't know how to tag it so they just grasp at every tag they can find, then everyone else blindly copies these tags when they show up as related tags for that character. At no point is anyone thinking about what these tags are for, they're just trying to maximize their tag count because more tags is always better, even when it means stretching the definition of tags until they're useless.
If you look closely you can see this happens all the time. Nearly every popular character will have some tag like this that's only used because everyone else is blindly copying it. When the character is popular enough the tag will be used on thousands of posts and at that point the tag dilution process is complete.