In Pokémon Pokopia’s story, Ditto finds its former trainer’s old Pokédex.
The discovery is important because it connects Ditto to the vanished human world. After finding the device, Ditto assumes a form based on its trainer and begins moving through the abandoned landscape in a human-like shape. The Pokédex therefore functions as a story link rather than merely a bestiary or gameplay menu.
Pokopia’s plot gradually explains why the world is empty. Logs left by humanity reveal that worsening environmental conditions made the landscape inhospitable, leading humans to leave for outer space. Because they could not take their Pokémon, they stored them in a large computer system designed to preserve them.
A Poké Ball is a natural guess because it is one of the franchise’s most recognizable objects, but the item described in the plot is specifically a Pokédex. The Pokédex is the clue that helps establish Ditto’s relationship with its original trainer.