I don't think that this tag,
character, is useless.
"Nearly every question is about some character". It's good word choice: "near" ...it`s not the same than all. I've made questions about characters myself and I like them: one question with over 5 votes on Hodor, from Game of Thrones, and others less popular questions on Gilfoyle, from Silicon Valley, and even on God, in Supernatural. Maybe vote popularity has to do with show/movie popularity, so that might not be such a huge issue. Just my opinion.
On the other hand they're tons of tags not related to characters, just some to mention: film-techniques, production, animation, soundtrack, title, effects, props, among the most used tags on site.
The second point: that "
character wouldn't work as the sole tag on a question" seems sound. However, keeping the tag even as a secondary one can help to do cross searches on a topic, such as different aspects of a movie. For example it's not the same between these two sets of tags:
- fight-club, props, ending, production
- fight-club, character, dialogue, analysis, plot-explanation