Fan Theories

Hounds on a Fox Hunt
This theory is relatively simple and concerns the slightly transparent Neon Fish that can be seen circling Joki early in the comic. Basically, those fish are like hounds on a fox hunt, meaning their job is find whatever their "hunter" is looking for. In this case, the hunter is the Giant Skull Goldfish. Using context clues, the reason why the fish lead the Goldfish to Joki is, because he is an outsider. The Giant Goldfish is like a White Blood Cell: going after and destroying foreign bodies. What will the Giant Goldfish do to outsiders that came into Aisho? It should be obvious since it is filled with Skulls.

The Mutant Theory
Nobody in Aisho is actually native to the area. People just come through random portals, and then mutate into other things, like the nurses and glitch people. the reason for that is that the city has a number of normal (albeit brightly colored) fish, and fish that attempt to eat or attack anything that appears human. These mutations are basically an attempt to change the person to make them less human.

Alternate Theory
As said before, nobody in Aisho is actually native to the city. Aisho was built by someone or something else and is basically now abandoned, or the things that built it died off. Anyways, the people that get into the city will mutate. These mutations could be based on gender (since the Nurses seem to be quite feminine, and all the Glitch People are wearing the business suits men wear in big cities), personality, or the area that the person took shelter. Basically, if someone took shelter in a hospital, they would become a Nurse, if they were in an office building, then they would become a Glitch Person.

To avoid mutating, one would have to keep moving, finding new shelter every night.

PARTIALLY DISPROVED:
Going from a small conversation I had with Niko, Glitch People are the victims of the Giant Skull Goldfish. They are NOT mutant humans that formed from sleeping somewhere.

The (Overdone) Purgatory Theory
This (very cliche) theory takes into account two things: The Finnish to English translation of "Joki", as well as the fact his clothes weren't dry when the Nurses them back to him. In case you haven't read the trivia for Joki, when translated from Finnish to English, Joki becomes River. Now, gears should be starting to turn right now, since his name means River and his clothes are wet.

Basically to avoid making this article long winded, Joki drowned in a river, was taken to a hospital in the real world so they could attempt to revive him, they couldn't and he woke up in the Purgatory version of the hospital Joki died in.

The Neon Fish are the fish Joki saw while drowning, their silhouettes being illuminated by sunlight passing through the water.

The Glitch People are forgotten souls, cursed to wonder purgatory for not being evil enough to enter "Hell" or kind enough to enter Heaven. Speaking of Hell, the Giant Skull Goldfish is basically Charon the Ferryman. Charon the Ferryman is a character from Greek mythology who would ferry the souls of the dead across the River Styx. In this case, Aisho's Charon just devours the bad souls, meaning "Hell" is the digestive track of a giant fish.

Also, Ace is a soul that has escaped from the Giant Skull Goldfish. His cause of death most likely has something to do with smoking, or high buildings, or possibly both. We've seen this guy in three pages so far, I'm doing my best to include him.

Ace's Mask
This is a more simple theory, based around why Ace wears a mask. You could think that its because he's a bad guy in Aisho, or maybe he's really messed up under the mask, but I have a different idea.

What if he wears the mask, so he doesn't look human? I mean, in "Still Dreaming", a blue dotted line appeared on Joki's neck. Why? Well, with all the headless humanoids around, I'd say its because the Giant Skull Goldfish only cares about his head. I imagine that the things designed to get rid of humans in Aisho, only go by faces; if they have a human face, then they will die, but if they wear a mask to hide their human features, then they should be fine.

Which is probably why Ace wears his mask.

Counterparts
Now, there isn't much backing up this theory, except things I've noticed.

Joki has a color scheme of grape purple, black, white, orange, gray and light blue. He is also a reference to the playing card, The Joker.

Jack has the same color scheme, with minor differences (the grape purple being a bit darker, same with the grays). He is a reference to the playing card, Jack of Spades.

Joki didn't keep his cool in a stressful situation, and Jack is being pretty damn chill while looking at Ace's body.

They have similar hairdos as well, which when mixed all together, makes me think of some characters being counter parts of other, since when there comes to cards, a card always has an opposite color (cuz, Diamonds and Hearts are red, and Spades and Clubs are black).