The in-game protagonist species in Pokémon Pokopia is Ditto.
The player begins as a Ditto that has transformed to resemble a human. This premise allows the character to interact with the world in a human-like way while retaining the Pokémon identity and transformation-based abilities associated with Ditto.
A Tangrowth is also important to the game’s opening setup, and the story begins with an encounter involving Tangrowth and the transformed Ditto. That can make Tangrowth seem like the protagonist, but the controllable central character is Ditto.
Pikachu and Eevee are famous Pokémon that appear throughout the franchise, yet neither is the player character species in Pokopia. The game deliberately chooses Ditto because its Transform ability provides a natural explanation for customization, tool use, and learning moves from other Pokémon.