Having Gangle as an NPC can make sense and not break with the message of the series.

I read too many reasons why Gangle could be an NPC (too many, and all of them make sense), besides being the least "human" human, since changing your feelings for a mask will never be reasonable. Beyond the debate, a human turning out not to be real would be a great plot twist, one that's been foreshadowed since the beginning of the series. Gangle is the one who always grapples with the dilemma of what's real and what isn't. Zooble had the answer for Gangle, but if it turns out that all her memories are false, it will add a significant conflict.