Hareta first chose Piplup to battle an angry Onix which had Mitsumi. Since Hareta did not have a clue about Poké Balls, He attempted to open it by biting it. This resulted with Piplup stuck in Hareta's mouth which had angered it and refused to fight, telling Hareta that he should fight instead. The Onix, after it had seen this, calmed down. Later on, in Professor Rowan's lab, Piplup fled to the forest, only to find the same Onix enraged for an unknown reason. Hareta and a forest Pokémon fought against it, moving Piplup. Onix was eventually calmed down with Piplup's BubbleBeam.
During their stay at Jubilife City, Piplup was stolen by B-2 because it was a rare Pokémon. Once it was freed by Roark, Hareta had his very first battle, against the Gym Leader. After a Combination of Pound, Bide, and an Oran Berry, Hareta earned his first Gym Badge.
Piplup later on helped Hareta catch a Shinx during his stay at Floaroma Town. Where it also battled against Mars's Pokémon during the invasion the Power Plant.
After winning 5 badges, they had arrived to Celestic Town, where they meet B-2 again after setting a bomb in the Celestic Ruins. Piplup used his Brine send the bomb rising in the air, where it exploded.
During Hareta's Confrontation with Cyrus, Piplup defeated his Weavile before being sent floating in the water by his Gyarados. Piplup was used during the battle at the Canalave Gym, before being sent by Byron to train in Iron Island.
At Iron Island, Hareta decided that he was relying too heavily on Piplup, so he decided to train without it. At Lake Valor, Piplup defeated Saturn's Rhyperior twice and alluded Cyrus's Gyarados's attacks. Piplup was used later on the battle against Mitsumi. It, along with Regigigas, had defeated Mitsumi's Electivire and Dusknoir were the only two Pokémon to be standing after the battle.
After the battle, Cyrus had the building self-destruct, with Hareta and Mitsumi still in the building. They were saved by Jun's Dragonite. However, Piplup was feeling sick and Mitsumi explained that before Hareta left for his journey, she attached an Everstone to Piplup, hoping that Hareta will learn faster with weaker Pokémon. She removed the Everstone, and Piplup immediately evolved into Prinplup and then into Empoleon, due to all that experience Empoleon got. Empoleon went with Hareta to Spear Pillar in order to prevent Cyrus from taking over Sinnoh with Dialga. When Dialga was about to attack with AncientPower, Hareta sent out Empoleon to prevent the attack from going through and then took on Dialga. Due to Dialga's ability to control time, Empoleon was not able to land a single hit on it and was finished by Dialga's Roar of Time.
At the Pokémon League, Hareta sent out Empoleon against Koya's Absol where Absol easily had the upper hand. As Empoleon was about to use Surf to counterattack, it was stopped by Mitsumi's Glaceon, since Hareta wasn't supposed to be battling in the stands. Empoleon battled Flint's Rapidash where despite the type-advantage, Rapidash defeated Empoleon with a combination of Sunny Day and SolarBeam. When Hareta got to battle Koya, Empoleon was sent out after Luxray and Regigigas were defeated. Empoleon began with Hydro Pump on Mamoswine which it missed since Mamoswine's Snow Cloak Ability made it faster. When Mamoswine was about to use one last Take Down, Hareta laid down on the ground to feel Mamoswine's movements and ordered Empoleon to dodge when Mamoswine got close enough. Empoleon defeated Mamoswine with a "Raging Rapids Hydro Pump". Empoleon was about to have its rematch with Absol until Team Galactic's return interrupted it.
In the Fight Area, Empoleon battled Palmer's Dragonite where both sides did their best but were defeated at the same time. After arriving in Stark Mountain, Empoleon used Surf to prevent Mars from getting hurt by Charon.
Hareta returned to Canalave City where he haa a Double Battle with Saturn using Empoleon and Minun. Since Minun only cheered instead of participating in the actual battle, Empoleon became outnumbered as Saturn's Magmortar and Gallade got the best of it. However, Empoleon turned the tables when Minun shocked Saturn and distracted Gallade. As a result, Empoleon was able to defeat Gallade and Magmortar with Surf.
Returning to Veilstone City and arriving in Team Galactic HQ, Empoleon battled alongside Koya's Absol against Charon's army of Pokémon. Empoleon attacked the gigantic Charon but it turned out to be a hologram of him. Empoleon than sat this one out for a bit until Koya has Hareta to command Empoleon to use Defog in order to get rid of the smoke surrounding the battlefield they were on. Things went well until Koya's Growlithe who was under the control of Charon arrived. Empoleon battled it and Koya jumped in when Growlithe made the first move as Koya did not want Empoleon to attack it. However, Hareta had an idea. He had Empoleon use Surf while Growlithe used Endure to prevent itself from being defeated. As the attack connected, Growlithe snapped out of Charon's control.
With Team Galactic finally defeated, the battle between Hareta's Empoleon and Koya's Absol resumed in the Pokémon League. As Empoleon went for the attack, it was not shown from this point forward who won. Empoleon returned home with Hareta after the Pokémon League was over.