From Bulbapedia, the community-driven Pokémon encyclopedia.
Tiffany (Japanese: アサギ Asagi) is a character of the day who appeared in On Olden Pond. She lives in Kanto by a lake, and she owns a boat house along with her grandmother, Galea.
For a brief time a Crawdaunt appeared in the lake and frightened the Pokémon and fishermen away. To get rid of it Tiffany offered fishing rods to travelers who visited the lake so they could try to catch the Pokémon.
With the help of Ash, his friends, and a wild Dragonair that lives in the lake, the lake became clean again and all the Pokémon came back, allowing for it to thrive once more.