Bond Phenomenon

The Bond Phenomenon is a term where through a bond with its trainer, a Pokemon can activate its ability which similar with Mega Evolution.

History
Not much known about this ability, but it said that the phenomenon once occurred several hundred years ago.

Ash-Greninja
One such example of the evolution is a Greninja which owned by Ash. During the battle against Heidayu, Greninja through his strong bond with Ash temporarily activates the Ash-Greninja mode and defeats Bisharp.

Weakness
Despite increasing in Greninja's strength and ability, it comes with the side-effect: If Greninja was attacked and takes damage, Ash will feel his pain.