The third evolution of Ralts(the evolution of Kirlia) it made its first appearance in the Hoen region. Gardevoirs can be male or female, but a male Kirlia can evolve into a a Gallade. They are known to be very loyal to their trainers.
Gardevoir has made a few minor appearances in the pokemon manga. A Gardevoir appears in the Pokemon Mystery Dungeon manga. In Mystery Dungeon it saves its human friend's life, but dies in the process.
Appearances in Other Media
Gardevoirs appear in Ruby, Emerald, Sapphire, Diamond, Pearl, Heart Gold, Soul Silver, XD, White, and Platinum they can't be caught in the wild(they have to be evolved form a Kirlia). They can be only to be traded in these games: Leaf Green, Fire Red, Colosseum, and Black.
In Pokemon Rescue Rangers Red and Blue Rescue Team: a Gardevoir visits the main character in his/her dreams. It is revealed she is the spirit of a Gardevoir that protected her trainer, but died in the process. She is revived later on in the game, but forgot her past life.
Pokemon Rescue Rangers Explorers of Time/Darkness: a Gardevoir appears on the rescue team Charm.
Super Smash Bros Brawl: she appears as pokemon that comes from a poke ball and also as a trophy.