Gardevoir is Diantha's only known Pokémon and is able to Mega Evolve into Mega Gardevoir. In The Bonds of Evolution!, it was first seen fighting Magnus's Absol in Brackish Town, where it used its speed to completely avoid taking any damage before defeating it, doing so without needing to Mega Evolve. Later, Gardevoir battled Pikachu, and was able to completely outmatch its opponent again in terms of speed and power. Before it could win, Gardevoir was captured by Team Rocket, who attempted to flee with it. However, Diantha followed and Mega Evolved Gardevoir, who easily escaped its captors and sent them blasting off.
In Championing a Research Battle!, Gardevoir battled Ash's Greninja. Although Gardevoir initially held the upper hand, it eventually had to Mega Evolve to keep up with Greninja's unique transformation. When Greninja perfected its new form, it overpowered Gardevoir, but passed out from the resulting strain. Gardevoir was then targeted by Team Rocket again, but it easily sent them blasting off. It then dropped to its knees in exhaustion; Diantha thanked it for a job well done and recalled it.
Being calm just like its Trainer, Gardevoir proved powerful in its debut appearance, being able to dodge attacks from opponents with and without verbal commands, also proving a powerful bond with its Trainer.
Diantha uses Gardevoir as her signature Pokémon, saving her for last and Mega Evolving her directly after she enters the battle. She is also on her Battle Chateau team after the player enters the Hall of Fame.
In PS575, Gardevoir was used to battle against Malva and her Braixen. Braixen starts the battle with Magic Room, surrounding them both in a giant sphere that isolates them from the outside. Malva notes that her plan was to use Magic Room to try and prevent Gardevoir from using Mega Evolution, only to find that move has no effect on Mega Stones. Despite this, Malva continues to order Braixen to furiously attack and trap Diantha. As they battle, the Magic Room begins to sink into the ground. Y and Shauna attempt to stop this, but are unable to break open the Magic Room barrier before it fully disappears.
In Yveltal Steals, Gardevoir battles against Malva's Talonflame. Due to Malva stealing her Mega Charm, Diantha is unable to Mega Evolve Gardevoir during this battle. The battle is later interrupted when Malva runs off to try and save Lysandre from falling to his defeat at the hands of X.