Piplup first appeared in Shock to the System!, where she was shown to be the protector of a group of Pokémon living near a lake on Route 203. When a wild Luxray attempted to invade the territory, Piplup battled it in order to protect her friends. However, she was no match for the Gleam Eyes Pokémon and was defeated after being struck by Wild Charge and sent flying into the lake. Lewis dove in to the lake to rescue her and was able to heal her.
Piplup declared her intention to battle Luxray again, with Lewis offering to help her train. Despite her reluctance to trust Lewis, Piplup agreed. The two then began training so that Piplup could learn Ice Beam in order to slow down Luxray's movements. After a whole day's training, Piplup was able to master the move.
The next morning, Piplup encountered Luxray again and challenged it to a rematch. This time, Piplup was more prepared for Luxray's attacks and was able to defend better. Piplup eventually managed to freeze Luxray's legs with Ice Beam and defeated it with Drill Peck. The celebrations were cut short when Kai arrived and stole several of the Pokémon living in the lake. Piplup, Aipom and Luxray then teamed up to defeat Kai and save the Pokémon. Following this, Piplup decided to go with Lewis, the Trainer having earnt her trust. Before leaving, Piplup asked Luxray to take her place as protector of the lake, which it agreed to. Lewis then caught Piplup in a Poké Ball and left for Oreburgh City.
Piplup was Lewis' third and final Pokémon in his Oreburgh Gym battle against Roark in Mine Away Home! She first battled Onix, after the Rock Snake Pokémon had defeated Murkrow. Making full use of her super-effective moves, Piplup was able to defeat Onix. She then battled against Roark's Cranidos, but was switched out for Aipom after a brief battle. Once Aipom had been defeated by Cranidos, Piplup was sent out once again. Countering Cranidos' Head Smash with Drill Peck, Piplup managed to freeze the battlefield to make charging around more difficult for Cranidos. Striking with a super-effective Bubble Beam, Piplup was able to defeat the Head Butt Pokémon, earning Lewis the Coal Badge.
Lewis used Piplup in a battle against Lucas and his Buizel in Pop Goes the Buizel! Both Piplup and Buizel made excellent use of the riverside battlefield, but the battle eventually ended in a draw. After the battle, Piplup and Buizel both affirmed their new rivalry.
Piplup is a proud Pokémon that takes battling seriously. She was shown to be very caring to the Pokémon living at the lake and would protect them no matter what. Piplup is quite distrusting of humans, as shown in her initial interactions with Lewis, although is prepared to ally herself with them if the situation arises. Indeed, it is shown that the only human Piplup fully trusts is Lewis, as she was impressed with his desire to help her and to protect the Pokémon of the lake, and will only accept food from him.