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I decided to reread and found something interesting about the cassette tapes. I noticed that the cassette tapes changes at times. Here they are:
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-171057/pg-3/
If you look at chapter 2, page 3, Hyungshin is holding a white and cyan striped cassette tape. In the prologue, he always inserts this into the player but no audio ever comes out.
Now in: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-198972/pg-25/
Look back at this chapter when Bada first teaches Doha about the game after the torture scene. Bada tried to hide the fact that the cassette is how to save the game:
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-173070/pg-3/
Note that the cassette tape is still white and cyan striped.
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-173070/pg-5/
In the same chapter, Bada finds a Silver Star cassette tape player on a low table in the middle of the room. He inserts the white and cyan striped cassette tape in and after Bada leaves the room, Doha stares at the top of the shelf near the door, implying he finds a different cassette tape player. At
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-174084/pg-11/
the flashback seems to show a black cassette tape player.
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-175168/pg-13/
Bada finds the black cassette tape player again in ch 9 page 13 but this time:
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-175168/pg-14/
he holds up a BLACK and cyan striped cassette tape. Note: this is AFTER Doha says that he inserted the tape into the black cassette player.
Also here: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-178006/pg-15/ and https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-179121/pg-5/.
Bada doesn't seem to notice though and Doha enters the BLACK and cyan striped cassette tape.
But in chapter 18 page 4, Hyungshin is now shown to have inserted a white and YELLOW striped cassette tape: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-185119/pg-4/
It continues to the white and YELLOW striped cassette tape: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-191179/pg-13/
Here, Doha find the black cassette tape player: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-193209/pg-22/ in chapter 25 page 22, where he seems to be fiddling around with.
This is the cassette tape that is shown in ch 30 page 2:
https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-198972/pg-2/
Here you see that Hyungshin enters a white and YELLOW striped cassette tape in the prologue (the reset after Hyungshin and his love interest escapes after Bada's "sacrafice" at the bridge)
But in Bada's memories, he remembers a BLACK and cyan striped cassette tape: https://www.mangago.zone/read-manga/dreadful_night/uu/br_chapter-198972/pg-16/ page 16.
In page 25, Pi Bada's cousin searches Doha's bag and finds two cassette tapes, the white and cyan striped one with a checkmark and the new white and yellow striped one and gets killed for it after thinking "hey these looks familiar?"
My theory:
(1) The color changing is on purpose. The white and cyan striped cassette tape was the original save tape or at least starting from the "19th" session according to Bada (Bada is an unreliable narrator tho, so idk if it's truly the 19th session in chapter 1).
(2) But starting from chapter 18, it's the white and yellow striped cassette tape that Hyungshin enters until season finale. The only one that could have taken the white and cyan striped tape was Doha, when he first enters the cassette tape into the black cassette tape player since he was left only with it
(3) This somehow "ended" the white and cyan striped cassette as there's a checkmark in season finale and Doha can somehow keep it (in his bag in season finale)
(4) This implies that the white and cyan striped cassette player is used for the whole team (excluding Doha) since Hyungshin entering the cassette tape also causes him to remember things, but note that Bada saves with the same cassette tape. Also note that in season finale, the cousin finds the cassette tapes to be "familiar"
(5) The black cassette tape is weird. Maybe it implies that Doha "corrupts" the save somehow? Or he "creates" a new black cassette tape when he first saves?
(6) Doha clearly knows more about the saves and cassette tapes because he keeps asking about it to Bada and is seen with them in his bag in season finale.
My guess: Doha is an "outsider" and the others are actually a team that somehow Doha keeps interfering with