What it does
Imposter fires the moment Ditto enters the field: it transforms into the Pokémon directly opposite, copying their species, all four moves (5 PP each), every stat except HP, their ability, and every stat stage change that's already been applied.
That last part is what makes it terrifying. If your opponent just used Swords Dance for +2 Attack, you come in with +2 Attack. If they ran a Nasty Plot, you copied that too. All the turns they spent setting up? Now working against them.
Two things to remember: Ditto keeps its own HP — you don't get the opponent's bulk — and the transform won't trigger if the target is hiding behind a Substitute. In doubles, it copies whoever is directly across, not your choice.
Who runs it
Ditto at 98% — basically everyone. Without Imposter, Ditto has no competitive reason to exist. The 2% running something else is experimental at best.
How to use it
Choice Scarf is the standard hold item, and the reason is straightforward: Ditto's base Speed is 48. After transforming, it uses the opponent's Speed stat — which is usually fine — but against anything faster, you're still second. Choice Scarf multiplies that by 1.5 after copying, which gets you ahead of almost everything. You transform into their sweeper and now you're faster than it.
The timing is the skill. You want to come in after the opponent has invested turns into set-up — right after a Swords Dance, right before a Nasty Plot user was about to start punching holes. You take the result of their work, and they lose their booster from the field in the exchange.
Watch your PP. Every move is capped at 5 uses. A long game or a move you need to spam can run you dry faster than you'd expect. Struggling is a real loss condition, not a theoretical one.
Tips & strategy
Mega Evolution steal: In Champions, if your opponent has already Mega Evolved via Mega Evolution and Omni Ring, Ditto transforms into the Mega form — getting the boosted stats and ability without paying any item cost. You essentially get the output of their investment for free.
Best timing to send it in: Late into the opponent's set-up, not early. A fresh, unchanged target gives you a 1-for-1 clone with 5 PP per move and Ditto's own (low) HP. A target sitting at +2 or +3 gives you all of that plus a free stat boost. The longer they set up before you click Ditto, the bigger your advantage.
The real weakness: Ditto's defenses are 48 / 48 / 48. If the opponent reads the Ditto switch-in and attacks instead of continuing to set up, Ditto takes a full hit before transforming. Don't telegraph it — you want the opponent to commit to another set-up turn before you come in.