Aya is a ninja and a Pokémon Trainer who specializes in Poison-type Pokémon. She is the younger sister of Koga, the Fuchsia City Gym Leader, and is a student of the Gym. Apparently, she was raised by Koga since she was young. She also has interest in poetry.

Aya made her first appearance in The Ninja Poké Showdown where they encountered Ash, Misty, and Brock in the Fuchsia City Gym. Aya challenged Ash to a battle with her Venonat and Ash agreed - sending out Bulbasaur. He won the match easily with his Bulbasaur's Leech Seed. After the battle her older brother Koga appeared.

Aya appeared again in Ariados, Amigos! where she was training at a nearby dojo, learning Pokémon Jujitsu. She and her Venonat were unable to master it; she even faced Ash and his Bulbasaur in a rematch and lost. But after an encounter with Team Rocket, she was able to bring out her Venonat's full potential and defeat them. She then returned to the dojo to fully master Pokémon Jujitsu.
Pokémon

(Source: Bulbapedia)