Marilyn Flame operates as a professional jewel thief across the Kalos region, targeting the Mythical Pokémon Diancie to harness its diamond-creating powers for personal wealth. She fiercely competes with rival thieves Ninja Riot and the Steel family duo, Argus and Millis Steel. Her trusted partner is a female Delphox, which employs Flamethrower against adversaries like Pikachu and Froakie.

Her hunt for Diancie ignites in Avignon Town, where Delphox blocks the Pokémon’s escape. Marilyn later confronts Ash Ketchum and his companions near an Orsay City fountain, battling his Froakie as the group flees with Diancie. Throughout these clashes, she exhibits a brash, talkative demeanor, contrasting with Ninja Riot’s silence yet sharing a competitive rivalry. The pursuit culminates in Allearth Forest, where Marilyn, Riot, and the Steels brawl over Diancie near Yveltal’s cocoon. Their struggle awakens the Destruction Pokémon, and during its rampage, Yveltal’s Oblivion Wing petrifies her Delphox and Yanmega. Overwhelmed by loss, she accepts solace from Riot moments before Yveltal turns both thieves to stone.

Xerneas later revives Marilyn, her Pokémon, and other petrified characters through Geomancy. This experience drives her to abandon crime. In the film’s conclusion, she works at a clothing shop, where Ninja Riot proposes marriage, cementing their romantic bond and her transition into lawful life.