Gardevoir in general, are not actually very aggressive in the wild. They mostly just teleport away whenever they see a human, if you catch one though they become very protective of the trainer and in their pokedex entry it says that they take extreme measures to protect a human they are attached too and can do pretty crazy things like making miniature black holes. So they are dangerous but not notably hostile, they usually get along with humans if they don't feel bothered.
