Pokémon Used FEELINGS! It's Super Effective!

So in Anistar City there's this house where an old man who lost his wife lives. He says that his life has become lonely since then, and asks if he can take care of one of your Pokémon, that must be level 5 or under. If you do so and leave one there, when you come back a few days later, you'll see an empty house with a Pokéball with your Pokémon inside and this letter.