* Rotom's role: In games like Pokémon Brilliant Diamond & Shining Pearl, Pokémon Sword & Shield, and Pokémon Legends: Arceus, Rotom is the spirit that inhabits the Pokédex. It's the one that unlocks the National Pokédex after you've completed the game's main storyline.
* Requirement for unlocking: You typically need to achieve a certain progress point in the game (like completing the Pokédex for the region) to unlock the National Dex feature. Rotom serves as the guide and unlocks the functionality.
Important Note: If you are playing a different Pokémon game, it's possible that a different Pokémon or event unlocks the National Pokédex.