"The next addition to Pokémon GO will be Buddy Pokémon. You will be able to pick your favorite Pokémon from your collection to become your buddy, opening up unique in-game rewards and experiences. Buddy Pokémon will appear alongside your Trainer avatar on your profile screen, adding helpful bonuses such as awarding Candy for walking together. You’ll also have the ability to change your Buddy Pokémon at any time."
Niantic Labs – Pokémon GO