Edna & Harvey: Harvey's New Eyes

Chapter 3

After finishing the RPG portion of the game, turn off Lilli's Get Angry restriction and talk to Peter twice. He will spit out a tooth and give you a character sheet. After this happens, talk to Peter 15-20 more times – each time, Lilli will slap Peter.

Unlike several trophies, you cannot skip the animation of Lilli slapping Peter. You must watch it every time.