Ninja Riot is one of the antagonists of Diancie and the Cocoon of Destruction. He is a ninja and thief who works against his rivals Merilyn, Argus Steel and Millis Steel to acquire a fortune through Diancie's power to create diamonds.

Riot appeared pursuing Diancie as she searched for Xerneas. He first attacked Diancie with his Greninja, also competing with Merilyn and her Delphox around Avignon Town. The two ganged up on Diancie and attacked her, but Ash intervened and told the two to stop. His Greninja was swiftly defeated by Pikachu, and Diancie escaped.
Later, Riot appeared near a forest, disguising himself within a tree. He attacked Diancie with his Greninja, and Ash and his friends attempted to fight it off. He revealed himself and battled Clemont's Chespin and Serena's Fennekin, but soon Ash intervened with his Froakie and Pikachu, and they fought off Riot and Greninja. As he attacked Diancie, however, Diancie's loyal Carbink servants interfered to protect Diancie. However, Millis Steel appeared and used her Chesnaught to fight off Greninja. Diancie then escaped once more.
After it received the power of the Fairy Aura from Xerneas, Riot and Merilyn attacked her in the Allearth Forest. However, Yveltal was awakened by the fighting and went on a rampage, during which Riot's Greninja and Ninjask were turned to stone by Oblivion Wing. He later found Merilyn crying over the loss of her Delphox and Yanmega to Yveltal. Having a change of heart, he removed his mask and offered comfort to her. Just as she accepted his affection, however, Yveltal aimed Oblivion Wing at them, and they embraced shortly before being killed as well. Later, the two of them were revived by Xerneas with Geomancy.
In the credits of the film, Riot — now dressed formally — visited the clothing shop where Merilyn had gotten work and proposed marriage to her.