Which Pokémon co-developer worked alongside Game Freak on Pokémon Pokopia?

Omega Force co-developed Pokémon Pokopia alongside Game Freak.

Omega Force is a development group within Koei Tecmo, and it joined the project because of its experience with sandbox-style games. Game Freak had created the Pokémon franchise and led many main-series titles, while Omega Force brought relevant experience in constructing open environments and building-focused gameplay.

The partnership helped shape Pokopia as a social simulation and sandbox rather than a conventional Pokémon role-playing game. The collaboration is also associated with Omega Force's previous work on Dragon Quest Builders 2, another title centered on construction and world development.

Several companies are connected with Pokémon Pokopia, which can cause confusion. Nintendo and The Pokémon Company published the game, while Game Freak and Omega Force developed it. Therefore, Omega Force is the specific answer when the question asks for the co-developer working with Game Freak.