Battle Bond is an event-exclusive ability introduced in the seventh generation that turns Greninja into Ash-Greninja. It is the signature ability of Greninja.
After a Greninja with this Ability causes an opponent to faint, it will change form into Ash-Greninja. In this form, its Water Shuriken's base power is raised from 15 to 20 and always hits 3 times. After his transformation into Ash-Greninja, even if Battle Bond is suppressed by Gastro Acid, Greninja will not change back to its original form.
If Ash-Greninja faints in battle, it will revert back to original form. Battle Bond will no longer activate, even if Greninja is revived and defeats another Pokémon in battle. After the battle, the Pokémon reverts to its original form. This Ability cannot be replaced by Role Play. This ability has no effect outside of battle