Those beta testers who were to become demons. The demon of them would do something that the other ten thousand players would not just naturally do for themselves. That being said, they would have three different forms of their player character within the game world.
The three forms are:
The Player Character
This is just their original player character form that they had designed themselves from the very start. How and why they designed them the way they did would remain the same as before. There would be no change to them, at least externally. Their in game world avatar would be changed internally, though, as the way they worked and functioned would be drastically different than before. Their status would immediately change, as it would no longer state their actual stats anymore, as those would all be increased to the limit, with their HP capping at one trillion. Their skills and abilities would be fully and completely maxed out as well. If there were an ability or skill in the game, they would now have full and complete access to it at the fully mastered level. Though, how would they, the nine of them, keep this all from the ten thousand players? That was where the system was helping them in that aspect, as there was currently no way for players to see their own status at the moment. Thus, no one could tell where their actual stats and skills were at the moment, just that they were going up as they were doing things, causing them to increase. However, this would always remain the case, as they would discover the means to access their status eventually. It was just something all the players had to do and discover for themselves within the game world. Log out of the game world, and they would only get a bare bones view of their player character's level only. All had that they were a player in their bare bones status screen as well. However, no beta tester was able to enter the room of another, as that was just not allowed. This was strictly enforced, as there was no way a person could allow someone else access to their personal room. The doors just shut way too fast to allow anyone else to come in with them.
In the game world, only the demons would have access to viewing their status in game, and they were also were given the conceal ability that made their stats look like they were the moment before they became chosen as a demon. They would also seemingly grow over time, like they were still a player as well. The players were not barred from viewing their status in detail, just delayed and needing to discover it. Also, conceal was not limited to the demons, it just that it and the counter ability reveal only worked as an intent for the user themselves as a one time thing. Only learnable for players once they were truly at what would normally be called the end game content, and only if they were stronger than the one they were using it on. If not, then reveal would not work properly, as conceal would be doing its job. This did not just apply to demons, but all beta testers as well. If the one using reveal was not stronger than the one using conceal, then it just would not, in general. This was not a method of discovering who and who was not a demon, though, as reaching end game content did not mean having end game levels, just reaching it somehow or way, then spreading the knowledge to everyone else. Thus, a user of conceal could only be covered by a user of reveal who was of a higher level than them by at least a single level. Since the levels of all beta testers varied too much, and there was another problem. The amount needed to use conceal and keep it active was basically nothing, but reveal was an insanely massive amount. Giving a choice does not always mean giving the means to abuse the choice as. Thus, they were not allowing players to be able to learn who the demons were, truly that easily, at least on their end. The Demon would be free to do as they want with their own status after being informed about how conceal and reveal work.
Also, they were just given maxed out level stats, and then the ability to properly adjust to them automatically. They had to learn how to control their newly given strengths and abilities on their own. Though they did not have to learn how to do this as their player character form, at least. That is where their second form came into play.
The Humanoid Form
This is a humanoid form of their full and true demon form. It's basically just a humanoid version of their otherwise massive kaiju form that they could assume at will. To fit whatever scenario they found themselves in, as there would be times when they would need this form, as they can not get by with just their player character and true demon form, after all. Though their stats, abilities, and skills are the same in all three forms. Using and/or revealing them while in their player form could oust them as a demon, which would be a mistake only someone thinking in the short term would make, as thinking only about the present would fully harm them in the future, after all. Thus, the demons that knew this used their humanoid form to keep players off their tail and used it to learn how to properly control and then blend back in with the crowds as a result. Things would be set up so that this would even be possible, as otherwise it would probably be harder for the demons to remain fully hidden if a key detail to their true selves was accessible to all. It would not be easy, but leave something that allows a person to locate someone else's exact location from a mere random photo. They would probably be able to discover the nine if something was not done to prevent that. Also, this was to keep even other demons from knowing their true selves as well, as they were not friends, teammates of comrades. Just nine beta testers chosen at random, and they would not already know each other, thus why would they automatically trust each other.
The Demon
This would actually be their true form, as their default form would be their massive kaiju sized demon self. Most players would assume that their player character or humanoid form is their true form. That is because they are beta testers just like them originally, but in reality, their demon form would be their true self in the game world. The other two forms would be alternate forms that they can take at will whenever they want. What they have access to in this form is not really that different than their other two that much, at least. The difference is that the scale of everything would be massive, given their size and weight. Some demons could become so paranoid about others that they never appear before others in anything other than their demon form. While others decide to appear before them in their humanoid form only. Then some never let anyone see anything but their player character form only.