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When the wielder comes into play, it instantly transforms into the opponent. Moves, ability, type, stats bar HP, EVs bar HP, stat stages and weight are replicated, but species, IVs, items, and gender are not. Moves copied via Imposter have 5 PP maximum. In multi battles, Imposter transforms the wielder into the Pokemon directly opposite the wielder. Imposter fails if the opponent has Illusion active or is behind a Substitute. Imposter only activates on switch-in; if it is acquired during the battle, it will not activate. If the wielder uses Entrainment or is targeted by Role Play, the move will fail.
Imposter is a godsend to Ditto, turning it from a gimmick into one of the most powerful revenge killers in the game. By giving Ditto a Choice Scarf, it is guaranteed to outspeed the opponent almost 100% of the time (barring the fact that the opponent may have their own Choice Scarf). In a metagame where many Pokemon are able to check themselves, Ditto can usually switch in without fear and start wreaking havoc on the opponent. While Ditto is still left with its subpar HP, the Speed it can obtain, combined with the power it can pick up from some opponents is definitely worth watching out for.
Found in Dream World