Merilyn Flame is one of the antagonists of Diancie and the Cocoon of Destruction.
She is a thief who wishes to get rich using Diancie's power to create diamonds, and the rival of Riot and Argus and Millis Steel in this goal.

Merilyn first appeared in the movie where she had her Delphox prevent Diancie from getting away in Avignon Town. She pursued Diancie as Diancie searched for Xerneas. Her's and Riot's attacks were loud enough that Ash heard them and ran over to see what was going on. As Merilyn was about to have her Delphox attack again, Ash appeared and Pikachu Thunderbolted her Delphox and Riot's Greninja. The impact of Pikachu's attack caused both thieves to flee the area. She was then seen in Argus Steel's computer when he was watching footage of the thieves.
Later, while Ash and his friends were in Orsay City, Merilyn found them by a water fountain. Ash kept her busy with Froakie while the group ran off with Diancie. She met up with Riot and Millis Steel where Millis distracted the two thieves long enough for the group to get away.
After Diancie was purified by Xerneas's Fairy Aura in the Allearth Forest, Merilyn, Riot, Millis, and Argus, confronted the group there and the thieves fought each other over who would get Diancie. Merilyn and the thieves took their fighting to Yveltal's lair in Allearth Forest. When the thieves stood on the water that lied Yveltal's cocoon, it became tainted where the cocoon started to hatch and Yveltal woke up from it.
During Yveltal's rampage in the Allearth Forest, her Delphox and Yanmega were turned to stone by Oblivion Wing. Despondent over the loss of her Pokémon, Merilyn was comforted by Riot, whose Greninja and Ninjask had also been affected. Just as she accepted Riot's affection, Yveltal aimed Oblivion Wing at them, and they embraced shortly before being killed. Xerneas later brought them back to life using Geomancy.
In the credits of the film, Merilyn gave up the life of a jewel thief for honest work at a clothing shop. Riot visited her at the shop and proposed marriage.

(Source: Bulbapedia)