Reckless

Reckless increases the power of recoil moves by 20%. By extension, this means the amount of recoil damage taken will increase as more damage is dealt (except for High Jump Kick and Jump Kick which are based on the user's HP).

Game descriptions

Diamond, Pearl, Platinum, HeartGold, SoulSilver:Powers up moves that have recoil damage.
Black, White, Black 2, White 2:Powers up moves that have recoil damage.
X, Y, O.Ruby, A.Sapphire:Powers up moves that have recoil damage.
Sun, Moon, Ultra Sun, Ultra Moon:Powers up moves that have recoil damage.
Sword, Shield, B.Diamond, S.Pearl:Powers up moves that have recoil damage.
Scarlet, Violet:Powers up moves that have recoil damage.