Ok, I found the answer myself thanks to this thread :

https://github.com/gamestdio/colyseus/issues/59#issuecomment-403288409

Currently, room.send() is the best method to achieve what I described.

Hello,

I started working on a game using Colyseus. Currently, I have all players in one room and I am synchronizing the entire game state with all clients. Actually, I would like to share some of the information only with each individual player. For example, each player needs to know only their own health points, not the health points of other players.

I am aware of the room.send() method, but it would be much more convenient if I could have automatic synchronization like it is possible with the game state that is shared with all players. Is this possible?

Thanks for your help!