Yapper is a character appearing in Layton's Mystery Journey: Katrielle and the Millionaires' Conspiracy. He serves a similar role to the Hamster, the rabbits and Hazel.
When Katrielle and co. are in pursuit of Shadee, he tells Sherl that he saw him (Shadee) head down to the Wharf.
When Seán Butchin is inquired about his sightings of/relations to Ratman, he explains that one time Yapper had stolen his sandwich, and Ratman had to go get it back for him.
Yapper later brings this up again with Sherl, where he states that Ratman had 'showed up and ruined everything'.