From Bulbapedia, the community-driven Pokémon encyclopedia.
Mirror Serena (Japanese: 鏡の国セレナ Mirror World Serena) is a doppelgänger of Serena, and a character of the day who appeared in The Cave of Mirrors!. She is Serena's counterpart from the Mirror World, whose personality is opposite to that of the regular Serena, as she is arrogant and takes battling very seriously. In the original version, she speaks in a Kansai dialect.
Before the regular Ash stumbled upon the Mirror World, Mirror Serena had a battle with Mirror Ash, which she won. After this, Mirror Serena bragged about it and called Mirror Ash a crybaby. This caused Mirror Ash to run off and decide to travel by himself. The Mirror World counterparts of Serena, Clemont, and Bonnie then lost sight of him. They later found him again just outside Reflection Cave, but were not aware that this was the Ash from the regular world instead.
As both the regular Ash and the Mirror World counterparts of his friends started to notice the different personalities, Mirror Clemont deduced this was not the Ash they knew. Mirror Serena refused to believe this, and challenged Ash to a battle, stating she would believe it if he managed to win, which Ash accepted. Mirror Serena used her Fennekin against Ash's Froakie, but Fennekin was easily defeated. Stating she would never lose to Mirror Ash, Mirror Serena now believed this Ash was indeed an Ash from another world.
After the battle, Mirror Ash showed up, having watched the battle from behind a tree. He explained how he had lost his own Pikachu, and the group then decided to go look for him. They ultimately found out he was encaged by Team Rocket of the regular world. Team Rocket also managed to encage the regular world Ash's Pikachu. While both Ashes battled Team Rocket, Mirror Clemont snagged the cages and threw them towards Mirror Serena, who freed them with her Fennekin.
After they blasted Team Rocket off, Mirror Serena helped in getting the regular Ash back to his own world, as he needed to get back before sunset to prevent getting trapped in the Mirror World forever. They found the mirror through which the regular Ash had come, and managed to get the portal to appear. As the portals of the regular world and the Mirror World started to drift further apart, Mirror Serena encouraged her regular counterpart, who was at the other portal, to wish strongly for Ash to return. The regular Ash ultimately managed to get back, and Mirror Serena said her farewells as the portals closed.