Probably none of that, since a ring is made for a finger. It probably magically detects if it has made contact with one. Now, if the question was what if it was put on a severed finger, then that is something to think about

I think the idea is that the majority of the consumers and viewers of anime and manga are teenagers. To pull in that audience, the characters would have to be relatively the same age for the audience to relate to those characters.