From Bulbapedia, the community-driven Pokémon encyclopedia.
Rivalry (Japanese: とうそうしん Belligerence) is an Ability introduced in Generation IV.
When using a move, Rivalry raises the power of the move by 25% if the target and the Pokémon with this Ability have the same gender, but reduces base power by 25% if the target and the Pokémon with this Ability have opposite genders.
If the Pokémon with this Ability or the target is genderless, the move's base power is unaffected.
Outside of battle
Rivalry has no effect outside of battle.
Pokémon with Rivalry
In other languages