From Bulbapedia, the community-driven Pokémon encyclopedia.
Giselle (Japanese: セイヨ Seiyo / 優藤聖代 Yūtō Seiyo) is a school girl who attends a school located somewhere between Cerulean City and Vermilion City.
In the anime
Giselle is a character of the day in one episode of the Pokémon anime. Her Japanese voice actress is Yumi Touma and her English voice actress is Roxanne Beck.
In The School of Hard Knocks Ash, Misty, and Brock had met a schoolboy named Joe who attended a Pokémon Trainer's school called Pokémon Tech. Apparently, upperclassmen at the school were bullying younger or less experienced students under the guise of "tutoring". The one responsible for this program was Giselle, a pretty and popular know-it-all.
Giselle confronted and belittled Joe, Ash, and Misty. Misty, angry at Giselle and her stuck-up attitude, challenged her to a Pokémon battle. Giselle defeated Misty despite a disadvantage and continued to taunt the red-headed Gym Leader. Giselle then started to lecture her fellow students.
Ash got angry himself and argued that Pokémon training was more than types and levels - you need to be friends with them, too. Giselle laughed haughtily and called Ash a beginner. She teased him about having been on a journey for two months and only having two badges and three Pokémon. She then proceeded to note that Pikachu are an awful choice for first Pokémon, and that they're more fitting as pets for little girls. Ash and Pikachu were extremely insulted and angered, and challenged Giselle to a battle.
Giselle used a Cubone. Without being able to use Electric-type moves, Ash told Pikachu to fight back in any way it could. Pikachu then began to attack Cubone with a series of random physical attacks - biting, scratching, etc. Eventually Pikachu covered Cubone's eyes with it's own skull helmet and Cubone ended up knocking itself out with it's own Bonemerang attack. Giselle was shocked by the defeat and admitted that people can't learn through books alone.
Team Rocket then tried to raid the school, but Giselle led the charge against them and got all the students to assist in chasing the criminals away. Joe and Giselle reconciled, and Joe left the school to collect badges for the Pokémon League.
This listing is of Giselle's Pokémon in the Pokémon anime.
Giselle was one of the only girls that Ash found attractive enough to abandon his usual indifference towards romance. Though some might argue that Misty has also made him feel this way.
In the Electric Tale of Pikachu manga
Giselle also makes an appearance in the Electric Tale of Pikachu manga. She is apparently traveling through Kanto and catching high-rank Pokémon.