Diantha is the Champion of Kalos and a movie star. She has no specialty type.
She debuts in a Lumiose City café when she steps in to stop Y from confronting Lysandre as he threatens Trevor, Shauna, and Tierno. Lysandre backs down rather than face Diantha and her Mega Gardevoir, and after the kids leave the room, she talks to Lysandre about beauty. He leaves after getting a phone call, and Diantha speaks to Y and the others again. Y realizes she is speaking to the movie star Diantha, and her excitement draws a horde of Diantha's fans. At this point, Diantha excuses herself and escapes.
Diantha has strong bonds with her Pokémon.
Diantha and Gardevoir have a strong bond, strong enough that Gardevoir can Mega Evolve with the aid of the Gardevoirite and key stone.
Diantha stops Lysandre from threatening Trevor, Tierno, and Shauna. The two have a serious conversation about beauty afterwards.