Be on Alert for Thieves

After you have beaten Gardenia in her gym, she is so overjoyed, she would love to share her love for Pokémon with you and take you to her garden, situated just outside the gym. If you have the time, you may choose to follow her into the her garden. If you do, you will see a person all clothed in black, trying to jimmy open a window to the gym.

"Hey you! What do you think your doing!?" yells the Gym Leader at the top of her voice. The person falls from the ladder and mutters some expletive under their breath. They pull out a PokéBall from somewhere, ready for the fight.

You can only attempt this challenge immediately after you defeat Gardenia in her gym. You do, however have the option to decline her offer if you wish. If you don't decline, you better be ready to take on this strange person without healing.