I should have warned you that we are living on flat Earth where everything is plane.

Not

either of these.

Just plain (sorry ) plane flat.

Here comes chubby Charlie.
He can move not only back and forth but also from side to side as well, which means that he can now roam  around all over this beautiful flat plane. Not only that but he can see and know everything about Arthur and Betty. He has very much greater awareness. Still has his limitations he cannot move up and down because he lives on flat earth.