With the revelation in the manga that Kafuka was Dead All Along, this puts up the disturbing thought that every time Kafuka appeared, she was actively possessing one of the girls in the class without anybody realizing. This also means that since the viewer can also see Kafuka, by extension you have a piece of Kafuka inside you as well!
"Which Kafuka are you?"
Nozomu once had a dream where everybody is the opposite of his/her real life self (for example, Usui is cool and popular and Kafuka is pessimistic). In that case, it is possible that Abiru ,an alleged victim of DV, is an actual victim of DV in the dream.
But it's okay though, it was all a dream.
Concerning the Kafuka hivemind, there's a theory going around that Maria came across one of Kafuka's organs because she was really hungry...
In one instance, Kafuka comments that her father is such a kind and protective father because he insured her life. Becomes Fridge Horror when you remember at the start of the series, she also commented (in her own way) that her father got laid off from his job and often tried to commit suicide. Put two on two together... especially with The Reveal.
After the revelation in the manga that Kafuka was Dead All Along, a lot of the nonsensical symbolism in the show (particularly the opening and ending sequences) now suddenly makes sense as Foreshadowing when you realize that a lot of it can be interpreted as metaphors for Kafuka's true nature, as well as her ability to possess her classmates. It also explains how she is seemingly everywhere all the time.
Zan opening has a short sequence where all the girls are singing but only Kafuka's voice can be heard.
Episode 3 of Goku's opening has all the girl turning into Kafuka one by one. In manga, there's also a similar occurance. In "Seeing the True Color" chapter, Abiru's eyepatch falls off, she sees that almost everyone in classroom are Kafuka, including herself when she sees her reflection on a mirror. This is also the first time any of the girls realizes about the phenomenon.
Zoku's opening shows various body parts, hinting on how Kafuka donated her organs after her death
The original anime's opening has a lyric that says "アタシがいるよ気付いて… (you just need to realize that i'm here)". A subtle foreshadowing on how Kafuka's spirit possessing her classmates since the beginning
There's a LOT of foreshadowing in the anime (which made people wonder if Shaft was in on the whole thing), for example:
In episode 4 of the first season, Kafuka (while posessing Abiru) wants Meru to look into her left eye. Kafuka's left eye was actually transplanted to Abiru. She also has Abiru's bandage on her right hand.
Kafuka briefly voiced Meru in Zoku episode 13. Meru actually got her voice from Kafuka. Not to mention that Meru was teased to the point of refusing to speak because her voice sounds funny, presumably from the operation.
Chiri make a realistic-looking heart-shaped chocolate for Nozomu for Valentine. The organ which she got from Kafuka was her heart.
In the Hot Springs Episode, Kafuka is mysteriously absent after the girls are detoxed. This probably means they have been purified from Kafuka's influence. Kafuka returns right after they get toxins from eating junk foods. And since Chiri is nowhere to be seen after Nozomu drowned in the hot springs, we can assume that Kafuka is posessing her.
The outro for the first season has Nozomu chasing Kafuka on a boat crossing the Sanzu river (think Japanese Styx), hinting that she already left the living world.
Not to mention that the first season's intro and outro have the girls' faces concealed or looking away.
Also, in the manga, no single panel shows all of the girls at once, since Kafuka has to actively replace one of them. When she appears, one of the girls that was previously present would inexplicably vanish from subsequent panels.
In manga, when Kafuka paints for sake of it, she paints a very detailed car accident, the same accident that took her life.
Abiru mentions in the manga that she got her eye transplant from someone who died in a car accident.
The line early in the series "A meeting that was never meant to happen" about Nozomu and Kafuka takes a more darker meaning given Kafuka was supposed to be dead by then.
Chie says that every girl will eventually forget about Kafuka, and any of her action will be remembered as one of the girl's action. However, this would raise suspicion when you realize that some of the girls she possessed (like Mayo or Kaga) would have acted very out of character.
well, Kaga has mentioned that sometimes she suffers from drastic Mood changes sometimes feeling really happy and others feeling even more depresed than (to her) normal, while it at first it may seem as kaga suffering from Bipolar affective disorder(A.K.A being Maniac-Depresive) it could also being that her highs are she being possesed by Kafuka and her lows being possesed by one of the girls from the showa period who commited suicide