"Petscop Kid" = Tester not Child.

If Anna's message was in the game plus it was her nephew's present, it's likely that she played the game.

Yes, I'm saying she is AnnaBelle.

I'm also saying saying she is Tiara.

"I called you Tiara prematurely." Where and when did Rainer call AnnaBelle "Tiara" prematurely? In the very first letter to Marvin, where Marvin's wife tells him of Care's eyebrows. "Tiara says young people can be psychologically damaged after rebirth", how does she know this? She knows because she went through this Candice Newmaker-like rebirthing herself when she was Tiara Leskowitz, hence the life letter, letting go of her past life.

Rainer designed the game for Marvin, to discover Lina's burial site. However, during the creation of the game Rainer discovered how the game is able to capture button prompts and repeat them, in a way capture the AI of a living person. What if you could use this to regress people through psychological tricks and hypnotism and thus rebirth them rather than a physical, cruel and tragic way like the Newmaker case?

Thus Rainer redesigns the game with this intent and AnnaBelle tests it before Care does, she was to be regressed back to being Tiara but she quit thus the AI is malformed. Rainer locks it away. Marvin and AnnaBelle still exist in real life but AnnaBelle is now psychologically damaged in real life. Rainer killed himself as a result and the game found it's way to Jill and her family who probably adopted Care after AnnaBelle dies in 2004 and yes Care is Paul. Paul has now being regessed himself successfully via the game.

The three characters are literally The Family. Father, Mother and Offspring. Presumably Marvin and AnnaBelle are both now dead hence the continued notion that "recordings have the power to raise the dead" as these are the only two characters aside from Paul predominant throughout the series, it seems likely they have passed on. Why is she called Belle, besides from a being a nickname, I think it's a narrative choice in this theory so Paul wouldn't know it was his mother, if he only knows her by Anna, he probably didn't know who is father was until after the regression.