That is a valid question. Thank you for bringing that up! :) Someone has also nominated some characters' mothers.

This will probably be something we'll ask the community to discuss. If the character is mentioned a couple of times so that you have something to go on regarding their characterisation, I don't see why we shouldn't allow them. However, I'm not sure about characters whose existence we just conclude.

We have a couple of other questions we want to ask the community, and I hope the discussion post will go up soon. :)