Toppy is a character in Professor Layton and the Last Specter. He is Luke's assistant with his predictions.
Plot
Professor Layton and the Last Specter
When Luke finally reveals how he is predicting when and where the specter strikes, he shows Professor Layton and Emmy a small mouse called Toppy. Luke had noticed that the water level of the canals fell where the specter would next strike, and, using his ability to talk to animals, he sent Toppy off to check the water levels on a day-to-day basis. However, Toppy made some mistakes that resulted in not all of Luke's predictions being correct. Layton used this fact to reveal that Jean Descole, disguised as Luke's butler Doland Noble, knew where and when the specter would strike despite these incorrect predictions.
Spoilers end here