I agree with Steves assessment, that's the route I'd go.  If you need to differentiate the behavior of the character accounts I'd probably create a subtype of user, owned by the master account, then a 'login_as' type functionality that allows you to simply switch between accounts at the push of a button.

Sorry for the absence, I get aught in some day to day chores.

@Steve, I think a relationship between users is a god solution if t comes to use a user object for the characters. Finding the solution that needs the minimum amount of rewriting in a strong reason of this discussion.

The secrecy of the main user is not a concern. In small communities these things are already known.

And yes, the characters need distinct usernames, friendships and group memberships.

I will study the roles plugin, maybe it's have the most of what I need to deal with players and character user. The login_as solution would be the easiest way to implement but offers the worst usability in my opinion. I will read again the notifications plugin that I not really understand to find a way to deal with notifications.

Thanks again.

I'm not sure how it would be the worst usability, it would be just as simple as switching google accounts.  I doubt roles will help you in any significant way.