Piplup is Dawn's main battling partner, used more than any of Dawn's other Pokémon and has helped Dawn win her first two Ribbons in Sinnoh. Piplup seems to be very competitive; he will always try to overcome the odds and impress Dawn and get her fullest attention. He often wants to be the first one to start an event for Dawn, and also often tries to peacefully talk to enemies, rather than just attacking them, as seen for example in The Psyduck Stops Here!.
In Three Sides to Every Story!, Piplup fell in love with a Marill; however, a wild Elekid is also competing for the Marill's affections. When the three Pokémon get separated from Dawn and the others, Piplup tried to help Marill, but Elekid seemed to gain the upper hand. When Team Rocket appeared and captured Marill, both Piplup and Elekid work together to save Marill. Once Marill was saved, she decided to go with Elekid, and left Piplup brokenhearted.
Piplup was used in the appeal rounds of DaybreakContest, along with Buneary. They managed to get Dawn to the battle round.
In A Grand Fight for Winning! Piplup was used along with Togekiss in the finals of the Sinnoh Grand Festival against Zoey's Gallade and Glameow. Despite coming up with a new combination and trying their best they lost at the last second.
In Memories are Made of Bliss!, Piplup threw a tantrum and ran away from the group when he realized Ash, Brock and Pikachu were leaving since he couldn't accept the fact. However Dawn and everyone else consoled him saying that they would always be friends to the end.
In SS024, he was used to battle Johanna's Glameow and Umbreon alongside Cyndaquil. But when Piplup and Cyndaquil unleashed their Whirlpool-Flame Wheel combination, the battle was stopped due to them fighting.
In All for the Love of Meloetta!, Piplup came to Unova with Dawn; both were able to reunite with Ash and meet Iris and Cilan in the next episode. During this time, Piplup was able to meet his new object of affection, a Meloetta that had been following Ash's group since Virbank City. He also found a rival in Ash's Oshawott who also had feelings for Meloetta. Later on, Piplup battled Cilan's Pansage and his dazzling attacks, including his new moves: Drill Peck and Ice Beam, left Meloetta greatly impressed. He also socialised with all the other Pokémon when they were brought out of their Poké Balls.
In Goodbye, Junior Cup - Hello Adventure!, Piplup grew sad when saying goodbye to Meloetta before he and Dawn left for Johto. His rival Oshawott then confronted him one more time, their contempt soon turned into sadness and they bid a teary farewell, before Piplup jumped onto the plane and departed with Dawn.
Piplup initially would not accept food from Dawn; as described in the Pokédex, Piplup are very proud and dislike accepting food from people. However, he seems to have a large appetite for Pokémon food and does not like having any taken away from him. He shows signs of having a short temper, frequently getting into disagreements with other Pokémon and his Trainer. One of his distinguishing traits is his tendency to lose it when things go wrong, such as when Dawn said that Ash's Gible did not have ill intent when he was practicing his Draco Meteor in Piplup, Up and Away!, when a Chimchar stole the last of his food in Following a Maiden's Voyage!, or when it repeatedly missed out on eating a berry and starts a fight with a Shinx in The Rise of Darkrai.
In A Breed Stampede!, Piplup was shown to be quite adamant and overconfident. He even tried to stop Piloswine from charging. However, each attempt resulted in being launched into the sky and knocked out. Despite this, he would get straight back up on his feet even when completely knocked out. He tried again when Piloswine had already evolved into Mamoswine, but the result was again the same.
Dawn's Piplup always seems like he wants attention off everyone, this may be due to the fact of all the Contests he performed in.
Bubble Sphere was used in A Grand Fight for Winning! it was the first attack made by Dawn, it starts off with Piplup using BubbleBeam and it forms into a circle and then Togekiss uses Aura Sphere in the middle of the circle. It is unknown what Dawn was going to do with this, because Zoey destroyed it with her Glameow and Gallade in a matter of seconds.
Aura Whirlpool was used in the final battle of the Sinnoh Grand Festival against Zoey. For the move, after Piplup launched a Whirlpool Togekiss would then fire an Aura Sphere inside, causing the Whirlpool to turn blue. Using this move, Piplup and Togekiss were able to successfully counter Glameow and Gallade's Signal-Tail combination.
Bubble-Swift Combination was a combination Dawn worked on during her time at home prior to SS024 it starts off with Piplup jumping on top of Cyndaquil and forming BubbleBeam into a circle then Cyndaquil uses Swift in the middle of the circle. It is unknown what Dawn was going to do with this, because Johanna destroyed it with her Glameow and Umbreon in a matter of seconds.
Flaming Whirlpool was a combination Dawn worked on during her time at home prior to SS024. For the combination, Piplup would cover a Flame Wheel created by Cyndaquil with a Whirlpool which created a sphere of water with fire inside. However, when Dawn attempted to use it during her practice battle against Johanna, the combination failed, throwing both Pokémon to the ground.
Big Bubble was a move Piplup used to fight off the Onix in Expedition to Onix Island!. Piplup used BubbleBeam but instead of multiple bubbles, it was one big bubble that surrounded Onix's head much like Water Pulse would. This caused a lot of damage and knocked the Onix out.
He is first shown when Dawn sends her Pokémon out to play with several other Pokémon. Later, he is used in to battle against Darkrai alongside with Buizel but they are both defeated by the great power of the Dark-type Pokémon.
The opening sequence of Pokémon Diamond and Pearl shows Dawn's Piplup coming out of his Poké Ball with a trail of stars behind him. However, Dawn did not use a star seal with Piplup's Poké Ball until Yes in Dee Dee, It's Dawn!, three seasons later.
Dawn has been keeping Piplup outside of his Ball, instead of inside of it, much like Ash keeps his Pikachu, though Piplup never directly expressed that he did not like being inside of the Ball.
Piplup was unwilling to obey Dawn when they first met. Eventually, they became fast friends after Pikachu and Piplup saved their respective Trainers from an attacking group of Pokémon; Pikachu saved Ash from a flock of Spearow while Piplup saved Dawn from a swarm of Ariados.
Also, he has expressed reluctance to evolve and chose to stay in his base form for the time being, and Dawn was given an Everstone to prevent Piplup from evolving.
During an interview with former director and storyboard artist of the Pokémon anime, Masamitsu Hidaka, he revealed that Dawn's Piplup would likely not evolve very soon, referring to his evolutions as "ugly" or not good for Contests.