Nia is the players advisor. She first appears when you start the game and from there, introduces you to the mechanics of the game via the dojo. She interacts with characters for the player as they progress through story mode. Her outfit can also be changed by the player. She trains a Weavile. Her frequency of dialogue can be changed in the options menu, as well as the ability for her to speak either Japanese or English.
Nia herself functions as a mechanic. In between every match Nia can cheer for the player, granting them a special bonus. This bonus can be changed by the player in the options menu. If the player has just lost a round and it's the final round then her cheer will max out both Support Pokémon's support gauge, regardless of the cheering ability the player has chosen.