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If the last amount of damage done before the use of Counter is greater than 0 and was dealt by a Normal- or Fighting-type attack, Counter will do twice as much damage to the opponent. Although Counter is a Fighting-type attack, the amount of damage it inflicts is not influenced by resistances or immunities. However, Counter's use is affected by accuracy and evasion modifiers.
Counter can be used as a set damage attack if the user has not been attacked by a move that deals any damage. So, for exmaple, if you successfully use Counter and deal 200 HP the first time, you can use it again the next turn to deal 200 HP to your opponent's Pokémon as long as your Pokémon has not had any new damage dealt to it, regardless of what type of move it is. Counter will only counter the last hit of a multi-hit move and the last attack of a partial trapping move. Counter will always miss if called by Metronome. An attack absorbed by a Substitute can be countered for the amount of damage it would have done to the user had the user not had a Substitute.
Counter is a niche move; it's a attack that makes the user goes last and can deal massive damage if used correctly. So, it requires some degree of prediction and the surprise value to work on most situations. Chansey and Jynx are the best users of this move, being able to deal high amounts of damage back with it, since a Tauros can try to Body Slam or Hyper Beam them to death.