I think I want clear enough in the rules that the unknown is not also a normal player. But aside from that, it gives them an opportunity to set up asymmetric information between the characters that plays to their motivations.
I.e. The unknown talks to one player, explaining their are a deity who wants them to perform a ritual to return the sun. But this isn't the truth. So that one character will be acting in a seemingly weird way under false pretenses.
Without that chance to work the players, interesting roleplaying like that wouldn't happen. IMHO