In Diancie — Princess of the Diamond Domain, on her way to find Xerneas, Diancie was resting in a house when she heard a Pokémon crying out for help. She decided to follow the sound to see if she could be of assistance. In the nearby forest, she found a Pangoro stuck in a hollow tree. She tried to get it out, but to no avail. Then, her loyal Carbink servants arrived and helped in getting Pangoro free by stomping down on the tree. Right at that moment, a group of Ariados appeared and threatened Diancie and the Carbink. The Pangoro they helped stood up, however, and drove the Ariados off. Later, Diancie and the Carbink set off in the sunset, unaware they were being spied on by Millis Steel.
After this small adventure, in Diancie and the Cocoon of Destruction, Diancie arrived in Avignon Town. Here, she was attacked by Merilyn and Riot along with their Pokémon. Ash was able to save her after he heard the two thieves attacking her. While Ash distracted them with Pikachu, Diancie was able to get away with Ash and his friends. She spent time with the group and found out that she has the ability to make diamonds. Diancie in turn learned about the outside world from them. During dinner, Team Rocket snuck in with a sack while Diancie, Bonnie, and Dedenne were getting food. They nabbed Diancie with the sack. She was then taken to a clock tower where Team Rocket asked her to make some diamonds. Diancie was able to do so and while the trio was occupied with them, Diancie ran away. She was unable to open the door of the clock tower from the inside. Millis Steel then opened the door for Diancie, setting her free. As such, Diancie was reunited with Ash and his friends.
Diancie wearing a dress
They headed to Orsay City by ship. Arriving in Orsay City, Diancie and the group go to a shopping mall. Since Diancie had not shopped before, Serena and Bonnie learned her about shopping by trying some clothing on her, while Ash and Clemont waited outside by a water fountain. Diancie's loyal servants Merrick, Bort, and Allotrope found Diancie in the mall and gave chase, reminding her of the task she was given. Ash was able to rescue her with one of Clemont's inventions.
Outside, Diancie got attacked by Merilyn again. While Ash held Merilyn off, Diancie escaped with Clemont, Bonnie, and Serena to safety. Riot arrived and tried getting Diancie with his Greninja's long tongue. Diancie was temporarily freed after Dedenne used Nuzzle on Greninja. The Carbink servants arrived and explained to the group their role as bodyguards to Diancie as she is their princess. From then on, the Carbink servants went with Diancie and the group. They arrived in the Allearth Forest where Diancie was able to meet Xerneas. Xerneas gave Diancie some of its Fairy Aura, which allowed her to successfully make diamonds without them disappearing.
Diancie and Xerneas
Millis and her father, Argus, arrived with Millis showing her true colors as she was only helping Diancie master her diamond powers so they could actually steal her once she received the Fairy Aura. Before they could do so, Merilyn and Riot arrived and Riot was able to snatch Diancie. She was taken to an underground pool where Yveltal's nest resided. The fighting over Diancie caused Yveltal to wake up from its nest in rage, causing it to destroy the forest with its Oblivion Wing. Diancie protected Ash, his friends, and Dace from another Oblivion Wing by Mega Evolving into Mega Diancie. As she Mega Evolved, Diancie was able to create a large diamond, which prevented the Oblivion Wing from hitting them. Xerneas arrived and stopped Yveltal from damaging the forest any further.
With the crisis over, Diancie, the group, and the Carbink servants returned to the Diamond Domain. Diancie Mega Evolved and restored the kingdom by creating another Heart Diamond. Bonnie reached into her bag and noticed the diamond Diancie gave her did not disappear unlike the other ones before Diancie was given Fairy Aura. Bonnie then gave Diancie the diamond to remind them of their friendship.
Personality and characteristics
Diancie and her Carbink servants
Diancie was shown to have very formal manners similar to those of royalty. This was especially seen as she often spoke to Ash, his friends, and Team Rocket in a very formal manner. While Diancie has good manners, she spent most of her life in the Diamond Domain and as a result was quite unfamiliar with the outside world.
Diancie was born with the power to talk to others through telepathy, namely her Carbink servants, and did not know what a thief was until Ash and his friends told her. She seems to have a good relationship with the Carbink that live with her, although they tend to get annoyed with her from time to time, such as Dace when she wouldn't take him seriously.
Diancie is incredibly generous, which was seen when she made diamonds for Team Rocket even though they had kidnapped her. She is also sensitive, which was seen when she was crying after seeing the Diamond Domain in ruins. She even went as far as saying that she did not deserve to be called a princess. After she saved the Diamond Domain, while she retained much of her personality such as her formality, she learned to take her duties and responsibilities more seriously.