Gardevoir is a Pokemon that appears as a spirit, and guides the player in his or her dreams, giving him or her advices. She is revived later by Gengar and the player.
A long time ago, a human and its partner Gardevoir entered a sacred dungeon, greedy of the rare Pokemon living in it. They encountered a Ninetales, and the trainer grabbed one of its tails. Ninetales tried to cast a curse on that human, but Gardevoir sacrificed herself and absorbed its curse. Ninetales asked the human if it wants to save Gardevoir, but it had already fled, leaving Gardevoir alone. Ninetales then said that the human shall turn into a Pokemon one day, and will make the balance of the world upset, bringing many natural disasters.
This Gardevoir is the same as the one in the legend.
Gardevoir appeared early in the game, when the player first had his/her strange dreams, although it is only as a soft, faint voice.
As time goes by, the voice becomes clearer every time the player has the strange dreams.
The night after the player heard the Legend of Ninetales, she/he had the dream again. The Pokemon came again and revealed that she is Gardevoir, scaring the player as he/she knew that him/herself was a human. Remembering that Gardevoir as in the Legend of Ninetales came, the player starts to ask him/herself if he/she is really the human in the legend.
The rumor of the player being the selfish human in the legend is spreading, and the player needed to run away. After leaving from the Frosty Forest, Gardevoir appeared in front of the player, she told he/she that Ninetales lives on the peak of Mt. Freeze, and the truth could be discovered by visiting it.
When returned to the Pokemon Square, the player had another dream. Gardevoir told him/her that he/she had came in order to fulfill a 'certain' role, and when he/she finished the role, he/she will return to the human world. But she also says that it is not the time to know about the role.
After the player had beaten Groudon, he/she has another dream again. Gardevoir told him/her that he/she came to save the world from the falling meteor. The player exclaims that he/she can't remember saying that he/she agrees to come here, but realizes that it's his/her destiny to save the Pokemon world, although he/she knows that he/she must leave his/her beloved partner.
After the player defeats Rayquaza and destroyed the meteor, Gardevoir appeared in front of the player (he/she hadn't followed the other Pokemon that is teasing Lombre). Gardevoir tells him/her that his/her job as a Pokemon is over, and he/she will go back to the human world, then disappears. She doesn't appear again until the events of Murky Cave comes.
Gengar started to regret leaving Gardevoir alone, and asks the player to escort him onto the peak of Mt. Freeze. On the peak, Ninetales reveals that she is sealed in the depths of Murky Cave, and gives Gengar the Nine Tail Crest, the item used for releasing Gardevoir.
At the depths, Gardevoir is revived and released from the curse, but had forgotten about Gengar, her previous partner. When she introduces herself to him, he replied sadly.
The next day after Gardevoir is saved, she is found in the Pokemon Square. She thought long and finally decided to join the player's team. She joins at level 5.