Buddy System

Buddy System
Using the Buddy System, trainers can choose one of their Pokemon to be their buddy, which earns Candy for their pokemon buddy by walking a certain distance.When you choose your buddy, you probably want one that isn't already at Max CP / evolution.

You'll also want to keep in mind how rare your pokemon is, since their rarity determines how far you'll have to walk to receive your free candy.PGOIV has a great chart showcasing how far each buddy will need to walk before generating a candy.

Pikachu Easter Egg
Reddit Pokemon Go repository The Silph Road reports that the most recent Pokemon Go update adds a neat new Easter Egg. The new Buddy system allows you to select a Pokemon partner to walk alongside your trainer icon.

Image Courtesy The Silph Road

If you select Pikachu as your buddy and then walk 10 kilometers, Pikachu hops onto your shoulder, a clear reference to Pikachu's habit of riding on Ash's shoulder in the Pokemon animated series.