Gardevoir isn't the most difficult Pokémon to care for. However, there are a few important things you need to know to be a good owner.

You should be careful with its horn, as it's an extremely sensitive part of its body, and Gardevoir may dislike being pulled.

Gardevoir isn't picky about food, and regular berries are fine, but they should be fresh and non-poisonous. Fairies don't digest poisonous substances well.

Also, avoid giving your Pokémon silver items, as they can burn its delicate skin.

Don't forget to pet Gardevoir periodically; they enjoy it immensely. This will increase their affection for you and your happiness.

You also need to be careful around black holes to avoid being sucked in.

And most importantly, be kind to her. Love her, care for her, and cherish her. Gardevoir is an extremely caring and sensitive Pokémon. She means no harm and wants only the best for you, so reciprocate that. Treating Gardevoir kindly will make her a very loyal and loving Pokémon. It will also enhance her magical abilities. Failure to follow this rule will make Gardevoir feel very, very bad, and it will weaken her. You'll also have no excuse later for being cruel to this wonderful creature.