Piplup is a kind of Pokémon which has a strong self-esteem. It seldom accept food that people give due to its pride.
A male Piplup made its debut in Pokémon Diamond and Pearl, Episode 1,Following A Maiden's Voyage!. In the episode, Piplup runs off on a chase with Chimchar, who stole Piplup's food, which caused it to become horribly upset. They run off into the woods and, at that very moment, Dawn comes into the lab, where she discovered that two of the starters ran off. With this and the fact that one of these Pokémon might be her choice, Dawn volunteers to help find Piplup and Chimchar. She soon finds Piplup. Piplup, however, is not as happy to see her. Then a swarm of Ariados storm in after Piplup hits its nest in the middle of his and Dawn's fight. With quick thinking and help from Piplup's powerful Bide, Dawn manages to trick the Ariados away. But now, Piplup's hungry and Dawn just wasted most of her Pokémon food. But there were still two pieces left, which she gave to Piplup. At that moment Dawn befriended Piplup, and it was then that Dawn's adventure began. Piplup often shows its pride during the Anime by hitting its belly. Sometimes its arrogant behavior has caused troubles. Piplup learn a new move called Whirlpool and Dawn uses it in a Pokémon Contest, allowing her to pass the first round. Piplup is often seen resting on Dawn's shoulder or on her arms without going into its Poké Ball. Perhaps Dawn learned that Ash's Pikachu preferred not to be in its Poké Ball, so he also follows it. It could also be that Piplup, like Pikachu, just never liked being in its Poké Ball which is indicated by the way Dawn got it in the first place (because it left its Poké Ball).