TL;DR
In the debacle 10 years ago, both Shan Shan & Cheng were good friends, were a part of the project, and were kidnapped.
Shan Shan got severely injured / almost died / actually died then got brought back to life, but Cheng thought she passed away.
This was the cause of both Cheng's issues and Shan Shan's "problematic" backstory
I've shortened names to one word to make my life easier (the first time per character, I put their future shortened names in parentheses)
(Relatively) Grounded Ideas:
- When Jia Cheng (Cheng) gets sent back to 8 years old, he got sent to shortly before the project
- Cheng is extroverted and doesn't really have his "problems" (Ch. 130)
- Cheng is probably 18-ish, since Shan Shan (Shan) is almost is/is 18 (Ch. 3)
- Shan, Cheng, and Jia Xin (Xin) knew each other 10 years ago and both participated in the project (the direction of the story pretty much makes it certain)
- A toy (most likely Shan's) ended up in Cheng's room (Ch. 92)
- Young Cheng was really extroverted and likely would have been open to making friends with angels, like Shan
- Both Shan's and Cheng's memories don't contradict them being there
- Cheng had bad memories in the past, which probably triggered his emotional shift, which happened sometime at or after ~10 years ago
- "Have you forgotten what they've done to you? ... How could I forget?" - Xia Fan & Cheng (Ch. 129)
- Shan got injured and has amnesia of the entire time period
- "I ... suffered a head injury ... memories of that period of my life became kinda fuzzy" - Shan to Xia Fan (Ch. 130.1)
- Xin knew Shan relatively personally before meeting her in person (possible)
- "I couldn't believe it when I saw that photo ... that was quite a shock ... that red hair" (Ch. 8)
- Of course, the hair color might be a hereditary feature of a powerful family (like a Hapsburg chin, but better maybe?) and Xin could be referring to that instead
- Before Cheng had his trauma, Xin was pretty normal (stature [1], ability, not a criminal [2], etc)
- 1: In Xin & Cheng's picture, Xin is normal-sized, while now Xin isn't allowed to not be a child
- Of course, this picture could have been taken
- 2: "Does them finding out that you still have an ability to restore your body seem like a joke to you? ... Don't forget that this identity of yours is the one of a criminal" - Supervisor to Xin (Ch. 117)
- At some point, Xin "failed" at supporting / being a good sister to Cheng
- ["Harrasing" Cheng] is the only way we, brother and sister, can get along ... I'm no longer qualified for [saying that I love Cheng directly]" - Xin to Supervisor (Ch. 117)
Theory:
- Cheng & Shan were good friends, and were close when living together during the "New Eden" project
- Possibly close enough for Shan to come over to Cheng's room, but this isn't necessary
- Also close enough to take a shit ton of pictures that would go in an album
- When the kidnapping occured, Cheng & Shan got kidnapped together
- Shan got severely injured to the point where Cheng thought she died (maybe she did and got revived, IDK)—this sparked his abandonment and trust problems ("I can't rely on anyone, I gotta do everything myself") because he maybe:
- Didn't want anyone else getting hurt or killed in the future helping him
- Others tried to help him, he trusted them, they failed
- Felt that he had to become strong and self-sufficient enough to fend for himself
- Perhaps other authority figures around him (possibly Xin) abandoned the kids (him and his friends) or him specifically when everything went down
- Perhaps this is the source of Shan's "head injury"
- Xin was involved in the whole thing someway (and got herself branded a criminal & got punished by being midget-ized), and also failed to help/support Cheng adequately and lost his trust—they slowly started mending their relationship until it became kind of close again now
- Maybe Xin failed to save Shan? Maybe when she failed she got blamed for causing her "death?" That could be an explanation
Application
- Once Shan "died," Cheng didn't go in his room [A] because it reminded him of Shan—he went in as little as possible, kept everything that Shan had given him (like the toy gun)
- A: "There is so much dust in here ... It looks like it hasn't been used in a long time" - Shan (Ch. 106)
- He ripped up his album filled with memories of Shan because he didn't want to be reminded of her
- A few people have seen this album:
- Cheng, who hates how it reminds him of his "dead childhood friend" (Shan)
- Shan, who has amnesia and doesn't know what it is since it's torn up
- Xin, who knows everything
- Supervisor, who, after taking care of Shan for years (Ch. 35, but also in other chapters I missed) and also knowing a whole lot of who-knows-what, can piece together what happened, helped by prior knowledge
- Cheng became obsessed with his "waifus" because 2D girls can't abandon him by dying, running away, or failing
Interpretation
- The normally broke & stingy Cheng buys Shan food even while feeling uneasy (Ch. 1) because he sees his dead childhood friend in her
- The reason Shan's toy gun is in Cheng's room is because Shan gave it to him or left it in there at some point and he put it in his room, then never took it out
- The reason Cheng gets royally pissed at Shan for going in his room (Ch. 107) is because he doesn't want people going around "defiling" his memory of "dead childhood friend," and maybe also since he sees Shan mirrored in his memory and he doesn't like it
I'm pretty sure there aren't contradictions in the story, but if you see anything let me know
Things this theory doesn't really answer
- Anything related to Yin(g) Xi
- Not much related to Shan's high status
- Why Cheng has no wings and why he seems to be relatively powerless, or why going on the island gave him some demon ears
- Can Xin transform into her "normal" form in the human world? Cheng still has his tail and eyes but no ears or horns in the human world? I don't know how any of that works
- Young Cheng in his picture has pointy ears, slit pupils, and a tail, but no horns—did he lose the ears?
- Little form Xin still has pointy ears even while not having horns so I really don't know