This video about how the player sprites in Super Mario Bros 2 work is very neat, but also kinda creepy in the implications.

so the tl;dr is that in Doki Doki Panic & SMB2, the player sprites are made up of 4 8x16 sprites, right?
and the engine has 9 sets of sprite-slots which it can use for items and enemies.
But the NES has a limit of 3 colors per sprite, and the player sprites have 4.

and the way that works is that they actually use a 5th sprite, solely for the whites of the eyes. And that's fine, that's okay.

But the implications of how they implemented that are where it gets weird.

so instead of redesigning the item/enemy slots system, they just inserted the EYES as one of them.
So while Doki Doki Panic lets you have 9 items/enemies on screen at once... SMB2 only lets you have 8, because one of those slots is always in use: BY YOUR EYES

there's always an enemy on screen, they're just inside your head.

and if that wasn't weird enough, the way they implemented it had to be slightly weird because of priority reasons.
Now, you'd think they'd just draw the whites of the eyes over an opaque mario sprite, right?

NOPE! CAN'T!

because the way sprites work in the NES means the lower-numbered sprites are drawn on top of the higher-numbered sprites, and the player sprites are reserved near the top of the sprite list.
Since the eyes are an enemy/item sprite, they are lower

so the player sprite will always be drawn on top of the eye sprite.

That's a shame, but easily fixable.

See, you know why NES sprites only have 3 colors?
It's because they are using 2-bit colors, allowing 4 colors.
But one of them (color 0) is hardcoded to transparent.
That's how mario can look how he does instead of being a 8x16 rectangle

so they just give the player characters TRANSPARENT EYES, and you see the eye-sprites through them.

so, to summarize, even when the player character is alone on a screen with no enemies, there's always one there, and you can see that enemy through the holes where their eyes should be.

sleep tight!

and you might think "but wait, Toad doesn't have any eye-whites, is he the only member of Super Mario Bros 2 who isn't infected with the parasites?"

NOPE!

because as is explained in the video, while he has no eye-whites, he still has that omnipresent hidden enemy following him around.

It's just fully invisible in his case.

so I would propose that Toad is actually the final stage of the infection. He's probably where it started, and it's reached the final form, completely invisible, symptomless but still infectious.

You can see it in his whiteless eyes. He's dead inside.

the being known as Toad died long ago. This is just the corpse of Toad being driven around the world of Subcon by the dreamkiller within, like a gruesome marionette.

Perhaps his fungal nature made him more susceptible.

and how interesting that "marionette" starts with "mario".

with other characters like Mario you can still see the whites of their eyes. But it's not their eyes... it's a hole.

if we disabled that sprite, Mario looks like this.

which is creepy enough on its own, seeing the void through Mario's head, but it's worse when he's standing in front of something.
Then it becomes clear that it's not just blackness filling his sclera, it's a hole. And you can see the background through it.

And the video points out there's a time in normal SMB2 play when you can spot that the eyes are not natural. You don't even need to dissect the game to know this.

Because of how the game shuffles the ordering of the enemy/item slots to prevent priority issues (especially when there's more than 9 on screen at once), sometimes they flicker, because the eyes are getting shuffled later in priority than other items, or disappearing entirely

and an easy place to see that is when a character (other than toad) goes through a door.
The door is a sprite, and the slot-shuffling means sometimes it'll be higher in priority than the eye-monster.
So they shimmer for a moment...

even the devil cannot hide himself perfectly.

sometimes there's a hint of the true form within.

this would be even more subtle on a CRT of the time.
It's flickering at 60hz, with the relatively slow phosphors of the time. So it wouldn't be so clearly flickering white/red, it'd instead smear pinkish

cross-franchise (but same company) linking:
Whatever weird parasite the characters from SMB2 have? it's related to whatever happened during that one creepy cutscene in Twilight Princess.

so if you want to know what SMB2 was really like, imagine blank-eyed Mario screaming forever

BTW, it's very weird that toad has no eye whites.
You know why?
because he's the only character that doesn't NEED this 5-sprite trick to give him eye whites. He's already got white as one of his 3 colors.

So they could have easily moved a few pixels around and Bam.
slightly-cross-eyed eye-white-having Toad.

but they didn't, despite it being easy and despite having this added functionality to add eye-whites to player characters.

So we have to assume this was an intentional choice. They didn't want toad to have visible eye-whites.

they wanted toad to have this dead-inside hollow glare.
they wanted to warn us

BTW given that they already had this separate eye-whites system, I'm surprised they didn't add a routine to recolor it the same as the current character's skin every N frames.

You could have had a really cheap blinking animation, in 1988!

BTW, Doki Doki Panic (and to a lesser extent, SMB 2) is like that because it's was made for the Yume Kōjō '87 festival, which had a theme of masks.

Masks might be handy to keep parasites out of your face. Doki Doki Panic was trying to warn us, too.

So in conclusion, learn from Doki Doki Panic and SMB2:
Wear your mask, to protect yourself and others.

And if you meet someone who has holes where their eyes should be, don't trust them.