Piplup is a Water-type Pokémon and the central protagonist of the anime special Pokémon Mystery Dungeon: Explorers of Time and Darkness. He is unique among his species in that he was originally a human who, for reasons unknown to him, woke up on a beach in a world inhabited only by Pokémon and discovered that he had been transformed into a small blue penguin-like Pokémon. He is found lying unconscious by a timid Chimchar who is running away from Wigglytuff's Guild, and the two soon become friends. Piplup is initially shocked and confused by his situation, insisting to others that he is really a human and becoming frustrated whenever he is referred to as a Piplup. His denial is shattered when he catches his reflection in a floating bubble and later in a puddle, seeing clearly for the first time that his body has become that of a Pokémon.

His primary motivation throughout the story is to understand what has happened to him and why he turned from a human into a Pokémon. Because of this, he is at first reluctant when Chimchar suggests forming an exploration team, feeling that he must first solve the mystery of his transformation. Nevertheless, the two decide to visit Guild Master Wigglytuff in the hope of getting answers, and through a series of misunderstandings they are registered as an official exploration team named Team Poképals, receiving their own room at the guild. That same night, as a sleeping Chimchar falls against him, Piplup experiences a vision of the near future showing himself, Chimchar, and Sunflora buying equipment at the Kecleon brothers' shop, a vision that proves accurate the very next day. This marks the beginning of his life as an explorer, and the two teammates take on their first mission alongside Sunflora, traveling to Labyrinth Cave to obtain a Gabite scale for a sick Shinx. Piplup is nervous at first, but with encouragement from the guild he begins to settle into his new role.

Piplup's most notable ability is the Dimensional Scream, a power that grants him visions of future events. According to Grovyle, this ability works most reliably when a trusted partner is present, and Piplup's strong bond with Chimchar makes him well suited to carrying it. As a Water-type Pokémon, he also has access to water-based moves in battle, and as a member of his species he is capable of evolving into Prinplup as he grows stronger. In terms of personality, Piplup is not highly expressive, especially early on, and much of his character is shaped by the bewilderment and unease of being trapped in an unfamiliar body. He is earnest and inwardly focused, and he develops a deep and dependable trust in Chimchar, who becomes his constant companion. Over the course of the story, he grows from a disoriented outsider into a committed explorer, and together with Chimchar he becomes involved in efforts to protect the Pokémon world from destruction, all while continuing to search for the truth behind his transformation.