きずなへんげ (Battle Bond)

Battle Bond is a signature ability exclusive to Greninja that triggers a powerful transformation after securing a knockout. When Greninja defeats an opposing Pokémon, it transforms into its Ash-Greninja form, gaining significant stat boosts and enhanced offensive capabilities. This transformation cannot be prevented through abilities like Simple Beam or Gastro Acid, as Battle Bond is immune to being copied or replaced by any means.

The ability underwent a notable mechanical change in Generation 9. In Generations 7 and 8, the transformation into Ash-Greninja provided substantial stat increases along with Water Shuriken receiving specific enhancements—the move's power increases to 20 and it consistently strikes three times rather than the usual 2-5 hits. However, Generation 9 simplified the effect to provide direct boosts to Attack, Special Attack, and Speed stats upon knocking out a target, removing the transformation aspect entirely.

In competitive play, Battle Bond creates a snowball effect where Greninja becomes increasingly dangerous as it racks up eliminations. The consistent three-hit Water Shuriken in earlier generations made it particularly potent for breaking through Focus Sash users and Substitute strategies. The ability's restriction to being uncopyable ensures that only Greninja can access this unique mechanic, maintaining its identity as a momentum-based sweeper that rewards aggressive play and successful predictions.