Niantic announced they were updating the buddy system in Pokémon Go, and now you can feed berries to your buddy Pokémon. Doing so will increase their overall happiness and their relationship with you.
To feed your buddy, you need to open up the Pokémon Go application and then click on their face in the lower left-hand corner of the screen. From there, you’ll jump into an AR section where you’ll see your buddy Pokémon flying around where your phone is pointed in front of you.
In the lower-left, you’ll see a variety of berries you have on hand to feed them. Click these, and you’ll pull one hand to dangle in front of your buddy. You need to toss it like you would a PokéBall and throw it their face. It should vacuum up if you’re off by a little bit, but you can miss, so keep it close.
You’ll need to do this until the meter is full, for the first time, and then you can leave the interface. If this is your first time feeding buddy Pokémon, they’ll now join you in your Pokémon adventures from now. You should see them nearby your trainer on screen.
When your buddy is hungry, they will stop following you. When you want to feed them again, click on their icon, hit the Play button, and then throw the berries at them. You’ll want to do this to keep them happy. You’ll have an activity to make them full, which is not entirely explained.