This Ability effect may be in need of research. Reason:Because Rivalry affects power which is calculated for all targets, does Rivalry stack when using moves that hit multiple targets? If so, how does the stacking work? You can discuss this on the talk page.
When a Pokémon with Rivalry uses an attack, the attack's power is raised by 25% if the target and the user have the same gender, and is reduced by 25% if the target and the user have opposite genders. Rivalry also powers up confusion damage.