S07E21: Heartburn

When Billy starts taking pictures with a camera found in Grim's trunk, Grim tells him that is an Underworld's True-heart Camera, which shows the real essence of a person's heart. When Mandy takes Irwin's picture, it is revealed that he is actually half-good and half-evil. Irwin explains that although he was born evil, his dad taught him about love and respect, and he had a change of heart. Upon knowing this, and against her own wishes, Mandy also experiences a painful change, turning good and kind-hearted. Mandy and Irwin finally share a romantic kiss in this episode, but it makes Mandy revert to her old dark and sinister self, which was Irwin's true intention, as he likes her just the way she is.