Toppy is a field mouse in Professor Layton and the Last Specter, who helps Luke Triton predict when and at what location the Specter will next strike. It is later revealed that Toppy locates where the water levels are lower than usual in the town and this is where the Specter will next strike.
Toppy is a purple/grey field mouse.
Throughout the game, Toppy can be seen running through the streets of Misthallery, in which he can be tapped to obtain Mouse Badges. Gaining 10 mouse badges will allow you to unlock the Mouse mini-game after talking to Fische.
"Toppy is a bright little mouse who gathered information for Luke by sending his brethren out to the far corners of the town to examine the pipes. His is a very clean species of field mouse that can only be found in the Misthallery region."