The problem is that 'identity' is context specific. You're right that it is an artifact of consensus - but it is actually consensus in different groups for different reasons and at different points in time.
At the beginning of the story, the protagonist's identity _is_ prisoner 24601. At the end of the story, it is not. Identity changes with time, context and circumstances, and how that identity is asserted will also vary from group to group.
An interesting study on identity is the (book/play/film) Les Misérables. At various points in the story, the protagonist is a prisoner (#24601), a Frenchman (Jean Valjean), a factory owner (Monsieur Madeleine), and a community leader (Monsieur le Maire). The main antagonist in the story, Javert, is so strongly biased in his rigid system of classifying people, that he is unable to accept Valjean's identity as anything other than a criminal - prisoner 24601. The internal dissonance this creates when Valjean continues to defy this categorization eventually results in Javert's demise.
At the beginning of the story, the protagonist's identity _is_ prisoner 24601. At the end of the story, it is not. Identity changes with time, context and circumstances, and how that identity is asserted will also vary from group to group.