Create multiple user accounts under one, sub user/child user

I am looking at finding a way to have a site with a setup like this:

– Parent user account (the account each user will have, can create multiple “child” accounts under this
— Child user account (account nested under parent account)

The reason I want this is so I can build a site that allows users to create fake profiles as fictional characters. I am building something that helps authors with character development, so them being able to post comments and stuff as their characters is what I am looking to do. I can’t find anything like this, though.