(There is more to it but I can't find the right ones right now)

Headcannon: Atem was already able to become visible as a spirit to Yugi when they took the millennium treasure box back home to the game shop and not after Yugi completed the puzzle. This leads to Yugi having a 'real imaginary friend' as a small child (since he started on the puzzle when he was 8 or 7 years old)

I think it's a cute idea and would make for an adorable roleplay