• English / United States: Diancie
• Japanese / Japan: ディアンシー
• Japanese (Romanized) / Japan: Diancie
• Japanese (TL) / Japan: Diancie

Diancie is the princess of Diamond Ore Country. In order to gain the power to create a "Sacred Diamond", the Diamond Ore Country's energy source, she has set out on a journey to find Xerneas. Diancie can Mega Evolve into Mega Diancie after it created a Diancite.

On her journey, she befriended Ash Ketchum and his friends who helped her get away from the bandits that were seeking to capture her to make diamonds for them. They traveled with her in order to find Xerneas and became friends forever after their experiences together.

Once Diancie met Xerneas, it already knew why Diancie was there and granted her that wish. Xerneas used its Fairy Aura on Diancie so it would let it unleash its true power and create a Sacred Diamond.

Having completed her journey and learned to Mega Evolve, Diancie was able to save the Diamond Ore Country by creating a new "Sacred Diamond" to replace the old one that had died out.