Niantic/The Pokemon Company
The coolest secret related to this new feature happens when you select a Pikachu to be your buddy: After walking 10 km with Pikachu at your side, it will hop up onto your shoulder! This Easter egg was discovered by The Silph Road, a community of Pokémon Go players that use the game's data to crowdsource solutions to its often obscure innerworkings.
Much like the Easter egg that lets players control their Eevee's evolutions based on what they name it, this Easter egg is a reference to the Pokémon animated series. In it, Pikachu wasn't particularly fond of his trainer, Ash. However, after some time together, their bond grew stronger and he could often be seen perched on Ash's shoulder.
So, whether your Pikachu sits on your shoulder from the exhaustion of walking 10 km, or whether it's out of a newfound affection for you, it's pretty darn cute.