Thanks to Shinoda, I've come up with an explanation of a theory as to why the reader or "(Y/N)" The Walker cannot be identified clearly by other Creepypasta in terms of their gender and age. In other words, an explanation for everyone's usage of they/them towards (Y/N).

because it's just a boring informational theory type thing about why the pastas call (Y/N) they/them and why they can't determine their age or properly tell their face.

This term is also known as face-blindness as stated on the Wiki. However, in this story it's not necessarily that everyone suffers from this disorder.

We're going to determine that SlenderMan (being the all-powerful being he is) is responsible for others being unable to fully identify (Y/N).

In the story, (Y/N) has technically fallen into a cliché "chosen one" scenario, which is why they were snatched up to become a proxy in the first place. Their potential outranks those of the other four proxies in this story; The Leader(Hoodie), The Mask(Masky), The Chaser(Kate), The Proxy(Ticci-Toby).

Power is like an aura in this dimension, I guess, and that power can be sensed by enemies of the Creepypasta, such as almighty Zalgo himself. (Y/N) is considered powerful(eventually). Since (Y/N) is such a risk, SlenderMan took matters into his own hands and in a way distorted (Y/N)'s face (I was thinking probably their overall figure as well) so that no one can truly tell who they are or in other words, he gave everyone except himself some high level of prosopagnosia.

In this story, we altered the definition the disorder to go along with the fantasies of the story; meaning like instead of plain face-blindness, its an extreme distortion.

It's like some Creepypastas may think they see a guy, some may think they see a girl, their voice may sound like that of a child's or maybe a teen or almost an adult. In terms of their whole figure being changed, some may see them being slightly taller or a bit shorter. This completely strays from the actuality of prosopagnosia but it's just being danced around for the story, used in a fictional way.

They may see (Y/N) a certain way, but they can never truly tell who they are, so everyone resorts to using they or them for pronouns. Some may have an ideal for (Y/N) and use what they think they are. But they will never see (Y/N)'s true face or be able to tell their age.

Since this is more of a disorder that damages the brain to only disassociate faces, it really doesn't fit when adding in things such as distortion of height at things like hair, but as this is a fiction where stupid , non-realistic things happen, we could say the damage of the distortion makes some people see (Y/N) with different hair color/ length, different eyes, voice, skin, whatever.

All of this is to protect (Y/N)'s identity from enemies like Zalgo, and despite living with the killers, their identity isn't completely safe solid around them either. Their importance to SlenderMan is so great that even the very ones working beside them cannot tell who they're actually working with.

Only SlenderMan and you the reader would know, since you the reader are the one that's either using an OC or yourself or whatever, and that means (Y/N) truly looks like however you want them to look.

This is a bunch of complicated, unneeded shit but I thought to myself it was pretty interesting when Shin told me about it so I thought maybe it could fit into the story like a theory and change it up a bit.

But besides that, it's just something I made up, I really don't care what you do with the reader or how you want the others see them, this is kinda supposed to you or your OCs story so it goes how you want it to go.

Anyways, let me know what you guys think about prosopagnosia on (Y/N), if you've even read this far.