The Bond Phenomenon (キズナ現象, Kizuna Genshou) is a where a Pokémon can bond with its trainer to activate an ability which is similar to Mega Evolution.
Not much known about this ability, but according to Professor Sycamore, it said that the phenomenon once occurred several hundred years ago and rarely happens when the mutual trust between trainer and its Pokémon is high and they master it. Sycamore also noted that the phenomenon was related to the Pokémon's potential ability and it doesn't happen to just any Pokémon.
One example of the evolution is shown by Ash's Greninja. During the battle against Heidayu, Greninja's strong bond with Ash temporarily activates the Ash-Greninja phenomenon and allows them to defeat Bisharp before reverting back to normal.
According to Clemont, when Ash and Greninja have exact same feeling, Greninja's appearance will resemble Ash's. Serena even recalls Olympia have told them that Greninja will receive full power which would prevail over greatness.
In the games, this ability is known as Battle Bond. Its full capabilities and attributes were initially unknown, but it does give a significant boost in speed and power, while shrouded in a water torrent veil. In this form, the move Water Shuriken becomes more powerful.
In the anime, trainer and Pokemon also share each other's point of view. In his second form, Ash's Greninja has a full form that can surpasses even the strongest Mega Evolved Pokémon (even if it is a Pokémon that he has type disadvantage against) with the Water Fuma Shuriken on his back which can deal massive damage to, if not defeat the strongest of his opponents in one hit (Greninja was able to defeat Mega Sceptile with just one hit from Water Fuma Shuriken; despite the fact that water type moves have little effect against grass type Pokémon). It also turn his Cut attacks into blue kunai blades.
Despite increasing Greninja's speed and power, it comes with the side-effect; if Greninja was attacked and takes damage, Ash will feel his pain as they shares each others pulse, so if it is pushed too hard, Ash may collapse from exhaustion. It was soon revealed that the main reason Ash and Greninja were having trouble with this form is due to Ash not having complete faith and trust in his Greninja. It was resolved to be more bearable in a later episode.
- This type of evolution appears in the demo of Pokemon Sun and Moon, even though it was first made exclusively for the anime. However, Ash-Greninja also appears in the TCG.
- This type of evolution is similar to Synchro Evolution from the manga.