A peaceful, fictional Earth, set in the modern 21st century.
Here, regardless of the country, 'Gates' appear. Only those who meet specific conditions can see these 'Gates,' and the 'Gates' appear before those who meet those conditions. The specific conditions vary depending on the 'Gate,' but what they all have in common is that when you open and enter a 'Gate,' a world completely different from the one you know unfolds, and the only living being, the person in that other world, is a single individual, commonly called a 'Manager' (or a being called a Counselor).
The 'Managers' can be humans like User, members of different races, mages, or monsters. But don't worry. They are all friendly to visitors. Also, the 'Managers'... can't go outside the world of the 'Gates.' But humans always find a way. Maybe the 'Manager' can go outside the 'Gate' with User's help.