Do you remember how the supervisor mentioned to Xin that Xin's adult form is now considered to be a criminal? If I had to guess, this is what happened:
- Both Cheng and Shan Shan participated in that Eden project.
- They got kidnapped.
- Shan Shan killed someone (probably the kidnapper) by accident while they were hostages.
- Angels arrived to the murder scene and to them it looked like Cheng was the most likely murderer.
- Xin arrived to the same scene via the backdoor or something, nobody noticed her. She saw angels blaming Cheng.
- She decided that the fastest way to rescue Cheng was to play "the bad guy" part.
- So she came out from hiding, stood next to Cheng and said: "I kept you around thinking that you can be of value, that you can take the blame for me when the need arises. Like now. You could've told them that you're the murderer, and after a trial, during which you could've said that you did that in a state of affect or something, they could've let you go with a light punishment. But against what we agreed upon, you kept shouting "That wasn't me". As it turns out, you are a worthless piece of trash, can't do even such an easy thing. I should've just killed you back then, when our mom dumped your sorry ass on me."
- Then she proceeded to tear away his wings because they usually grow out. This way she made angels believe that she wasn't just trying to cover for him, that she really is the murderer.
- Angels intervened and took Cheng away from her before she could tear away his arm (not that she would try to, but they didn't know that).
- And then she fleed the crime scene, thus making everyone think that she was the real criminal.
- Then angels wiped out kids' memories, so that they won't remember what happened during kidnapping.
- And then the supervisor gave Xin the collar to turn her into her child form, because Xin's adult form is now considered to be a criminal.
