Pokemon GO‘s buddy system lets you walk around with your favorite Pokemon, and nets you some great in game benefits, too. With Pokemon GO‘s buddy system, you get to pick one Pokemon, and after that, simply playing the game as per usual will net you free Candies for that Pokemon. Thus, instead of having to catch a lot of one Pokemon to level them up and evolve them, you can simply continue playing the game as normal, and have the same effects.
Doing this is simple- touch the bottom left icon, and you can pick a Buddy Pokemon for yourself in the menus within. Pick any Pokemon you have caught- you’re not committed, as you can change your Buddy at any time. After this, just completing your walking goals nets you Candy. Remember, if you switch out your Buddy before you have completed your progress towards your next reward, you lose all the progress you had made- so the best point at which to change your buddy may be when you have already earned the latest set of rewards.
There is no limit to how many rewards you can earn from this system (though it’s obviously slow, since it makes you walk a certain amount with the game open before you get your Pokemon), and any and all Pokemon are eligible- so go out there, pick your buddy, and get back to catching them all.