A player can walk around with a favorite Pokemon.
A buddy Pokemon do not show up on the map, but its icon displayed by the an avatar's icon.
A buddy Pokemon picks candy by walking certain distance. Required distance varies by Pokemon.
Only one Pokemon can be a buddy each time.
Tap the menu icon at the bottom left corner of the screen.
Tap "BUDDY" button.
Choose Pokemon which you want to walk with.
Now the picked Pokemon is set as a buddy.
When a buddy Pokemon is deselected, walked distance is cleared.
What is DPS and EPS in Pokemon Go?
The best moveset ranking for Dragonite
How to use 'Poke Assistant' for team building