In Pokemon GO you can select a Pokemon to follow your Trainer while exploring the world. This is known as your Buddy Pokemon, and it’s the fastest way to power up your Pokemon. Go to your Trainer Profile and click the Pokemon next to your Trainer, or click the Pokemon icon next to your Trainer icon to assign a buddy Pokemon.
Originally, the only purpose of Buddy Pokemon was to collect candies as you traveled. Some Pokemon would find one candy after 1 KM, while other Pokemon might take 5 KM just to find a single candy.
As of January 2020, players can now interact with their Buddy Pokemon. Play with your buddy using the AR+ camera, feed them berries or poffins, take snapshots, rub their head, use them in a battle, and visit new PokeStops to increase their happiness each day. Become better friends to unlock bonus features, including an instant CP boost to your buddy!