Do you have any headcanons for how MissingNo would act in the Pokemon universe?
First I'm gonna take this opportunity to plug the Surferverse from my main - and especially the Professor Oak lore comic in particular, as it gives a lot of information, like MISSINGNO.'s origins and how it was given its name in-universe.
I suggest reading that comic before proceeding, because I'll be breaking it down and explaining it in more detail here.
My headcanons are admittedly a bit more meta than what you might be looking for, but I think the nature of MISSINGNO. is inherently meta, so it's more fun for me personally to try making its Doylist origins make sense in a Watsonian world.
I try to make the meta video game aspects a part of the in-universe lore - since Pokémon itself often makes subtle meta references to itself (such as including the Game Freak office itself). ORAS's Delta Episode, as one meta example, introduces the idea that each copy of Pokémon is its own timeline/universe, its own separate canon.
So for me, MISSINGNO. is a Pokémon that only exists within the timeframe of Red/Blue/Yellow.
Essentially: MISSINGNO. is the scattered remains of the Pokémon that never were. The mons cut from the beta that once did exist in-universe. But as the games were finalized, the universe rewrote itself to erase those Pokémon from existence - and from the memory of all trainers who used to have them.
MISSINGNO. became all that remained of them. Scattered memories, scattered bodies. They do not need to eat, they do not need to drink, they do not need to breathe, because they are now nothing more than creatures of light and garbled static, existing within the fabric of reality where nobody was meant to find them.
Professor Oak was one of the only people who noticed something had changed - Because he had initially programmed 190 slots into the Pokédex, but after the universal ret-con, suddenly 39 Pokémon's names had completely disappeared, leaving blank spaces. He could no longer recall the names or faces of those Pokémon, and neither could anyone else. He decided to hide those missing slots, and give them the error handling name "MISSINGNO." - Missing number.
But it wasn't until Bill asked him to test the Time Capsule that he actually discovered what one looked like. After Bill traded him a Pokémon from Johto that hadn't yet been discovered in Oak's time, its body fell apart and became a MISSINGNO.. This is the reason why Johto Pokémon are now banned from being traded in the Time Capsule.
Since Bill had no idea what a MISSINGNO. even was, Professor Oak theorized that MISSINGNO. would go extinct in three years time, being replaced by the future Pokémon of Johto. In the meantime, he worked will Bill to prevent any invalid Pokémon such as MISSINGNO. from being traded over to Johto, to hopefully prevent them from potentially existing in the future as well.
Professor Oak also started developing a secret feature to his Pokédex - a Rhydon error trap that would hopefully stabilize a caught MISSINGNO. and transform it into a Rhydon. But he had no way of testing if it would work properly, since he did not know if MISSINGNO. would start appearing in the wild.
But there is a part of MISSINGNO. that subconsciously remembers the world they used to live in, so they instinctively wish to return to it. And they found a couple entry points where the veil was weakest, and they could cross over back into reality - the shorelines of Cinnabar and Seafoam.
And they would eventually be discovered by the child prodigy from Pallet.
Of course, there are still many mysteries surrounding MISSINGNO. that would be impossible for anyone to understand. Like why it duplicates items by 128 upon being seen, or why it corrupts the Pokémon League's Hall of Fame website.
But it is ultimately harmless to the touch, tickling a bit like the warm static tingles of a CRT TV.
That is- until you try to view its information through the Pokédex. The Professor's error trap activates only partially, causing the MISSINGNO.'s form to become unstable, causing corruption to all that it comes into direct contact with. This corruption can be more painful, akin to that of an electric shock - but this effect is thankfully temporary and wears off after a short moment.
The MISSINGNO. can be returned to normal simply by scanning a different Pokémon with your Pokédex instead, and will go back to being safe and non-contagious.
Though MISSINGNO. does not need to eat or drink to survive, it can do so for pleasure, absorbing the food into its amorphous body.
Despite their odd physicality and no face to properly express their emotions, they do all still have their own unique personalities just like any other Pokémon!
But they can also be influenced heavily by their trainers and their perception of them. If you fear them, they will also believe they're something to be feared. If you use them as a tool, they may see themselves as such. But if you treat the MISSINGNO. you find with love and kindness, and it will return it back to you 128-fold.
I have a lot more to the story that's relevant to The Surfer and NaN specifically, but that covers pretty much all of my thoughts about MissingNo. in general! Hope you thought it was interesting!
