>>27590336Professor Sycamore did some research and found out that there's been a few Pokemon in the past that have done the same thing Ash's Greninja has done, and he's calling the instances "Bond Phenomena".
Best I can tell it's the result of a select few Pokemon inheriting a unique ability that allows them to do this, and they actively seek out a trainer that can draw out their full potential, and refuse to obey ones that cannot. Greninja's Bond form looks like Ash because Ash was the one who drew out Froakie's power as a Greninja, and Sycamore believes that there may only be one trainer uniquely qualified to awaken each Bond Phenomenon Pokemon.
