Belle is first mentioned in Petscop 12.
She looks exactly the same as the player character, but is likely seen as Quitter by other characters. Belle’s name is written in purple, same as Tiara’s (see Color Theory). It may be a reference to Candace Newmaker’s favourite doll.
Belle is first seen in Petscop 12 in the Quitter’s Room, which, judging by her 0 pieces count, may be her starting location.
Once the “Pyramid Head” enters the other half of the room she begins reflecting his movement, seemingly after getting teleported to match his position (the time on the clock doesn’t change, indicating that it likely wasn’t a footage cut).
This event matches perfectly with movements of Paul and Quitter in Petscop 10, implying the two occurences to be the same situation seen from different perspectives.
“Pyramid Head” then seems to lead Belle out of Quitter’s Room, possibly freeing her, and disappears when the two walk into each other. Afterwards Belle begins to explore under the Newmaker Plane, and periodically receives messages left for her by an currently unidentified person.
According to those messages she is now free, and has seemingly been running Petscop for 153822 hours (6 409, 25 days). Depending on when exactly the latter information was give, Belle may have been running the game since late 1999, June 2000, or sometime in between. The messages then ask Belle if she can perceive a room:
Are you still sitting on a chair?
Can you still look around the room? Is there still a room?
This may imply that Belle is a real player who became trapped within the game.
Further messages reveal that Belle isn’t a member of “the family,” and therefore her features aren’t included in the Child Library.
Belle eventually emerges onto the Newmaker Plane, where she sees Marvin. Marvin appears to be unaware of Belle’s presence, and repeats his movements from Petscop 11 demo. This may indicate that the two events are happening at the same time, but only Marvin is visible from both perspectives. Alternatively, Marvin may be following a pre-programmed path. It may also imply that the place where Belle sees Marvin is where the School may appear under the right circumstances.
Afterwards, while Belle is wandering the Newmaker Plane, further messages appear. First, she is wished a happy birthday, followed by a message that gets cut (only “I’m” can be seen). Later still, a more exposition-like message is seen:
I'm calling you Belle because that's who you are. You might be confused as to what happened. I was overeager before, and started calling you Tiara prematurely.
I created a space in the menu for you, still unused now. Then I put you inside the machine, and played the second movement of Stravinsky's Septet on the Needles.
I played it wrong, but that would have been okay. If you hadn't given up halfway, you would be Tiara. This is not what happened, and now I'm gone.
This seems to imply that Belle underwent a procedure that was supposed to turn her into Tiara - or possibly allow Tiara to be reborn as her - but she quit halfway through. Her being placed in Quitter’s Room may have been a punishment for that.
The second movement of Stravinsky’s Septet is the music which plays in Quitter’s Room in Petscop 7, while Quitter (presumably Belle) is briefly missing.
The space created for Belle/Tiara may be the last, empty slot in the Pets menu.
At the end of Petscop 12 Belle appears to bring up the game’s pause menu. It features a “demo recording” message, and lacks the “Quit Game” option, which may be another indication that Belle is trapped inside the game.