From Bulbapedia, the community-driven Pokémon encyclopedia.
The ghost girl is a specter that appeared in the anime episode Ghoul Daze!. She is an evil spirit that tries to capture humans and bring them to the spirit world.
She first appeared in the bushes, waving her hands and asking Conway to play with her. During the night of event, Conway followed her on the way to a cliff, but Dusknoir saves him. The ghost girl also lured Meowth with food, but Dusknoir appears to stop her.
The girl beckons Ash and Angie to follow her to the Summit Ruins. Then the girl tried to coax Ash and Angie into the mysterious cave, which is the entrance to the spirit world. Dusknoir prepares to attack the girl, but Ash and Angie battle Dusknoir to protect the girl. Dusknoir turns around to protect the two, and blasts the girl, and collapses the cave entrance, leaving the ghost girl in the spirit world.
The ghost girl appears in the book Pokémon Chapter Book #1: Diamond and Pearl: Pokémon Academy, a novelization of the episode.