So recently there has been the news of the new Professor Layton game and I am very surprised that a certain theory hasn't popped up yet. The theory of Simon Foster being Descole. I believe this completely because the Butler/Servant that has been with Descole the past few games, is with Simon in this game.
Now, some people might say that Descole could be using him to spy on the people conducting the investigation, but that is incorrect. Reason being, the way Simon stands is EXACTLY the same as the way Descole stands, and they have the same shoes, similar suit, and same smile. My last reason for having this theory is because in Last Specter/Specter's Call, Layton says he recognizes Descole from somewher, and in the newest upcoming game, Layton already appears to know Simon from somewhere before. So this is my theory on Descole's true identity, hope to see some other people post some other ideas soon!