Alright, theory time.
So far, we have seen 5 Chihaya
1 The suit one, who seems to be a veteran in the game who has gone crazy
2 The idol one, a newbie (also is the main Chihaya through the most number of chapters (3, 4, 6) )
3 The one in the sailor uniform
4 The one in the dark unifom (from the ghost chapter)
5 The one with the mask (whose left eye was slashed by the 2nd Ouka)
From the information spreaded out through the chapters:
- Chihaya apparently attempted suicide with a Suicide Parabellum, which drove her into a coma.
- In the coma world she is equivalent to God, where she has a good amount of control, but not total control.
- Chihaya is chasing her here in maybe an attempt to bring her back to the real world.
- In every stage, there is a count down, when it reaches 0 the giant bullet will appear and destroy everything.
- Everytime Chihaya uses the suicide parabellum, a new Chihaya is created. All the Chihaya can exist together in the same world, and each time they jump they jump to a ?random stage.
- The mask Chihaya exists as some kind of cleaner/executioner for reason unknown.
- In chapter 4, the train they ride is named Samsara, which is the cycle of reincarnation. That's why it's running in circle and has no stop, and also why Chihaya and Ouka discussed about Ouka's various lives, as if part of the reincarnation process.
My theory:
1) Ouka attempted suicide and is in coma, and Chihaya is coming to get her back. However, she doesn't want to return because she enjoys the time they are spending together/Chihaya chasing her, so she kept creating new stage.
2) Similarly, but factor in the giant bullet we may have some sort of Inception scenario. If the time for a stage runs out and the bullet reaches Ouka dies, so the main (original) Chihaya needs to find Ouka/convince her before that, and bring her back intact. If she cannot, Ouka has to use the suicide parabellum/executioner Chihaya has to kill her. With Ouka dying they can reach a deeper part of her consciousness where time flows slower, so they can buy more time and rinse and repeat the process until main Chihaya can rescue her.
The only thing i'm not sure about is what is Marrik. Is he a manifestation of Chihaya, since he has the same ruined left eye as the executioner Chihaya?