Imposter

Imposter automatically transforms the Pokemon into the opponent as soon as it enters the battle. The transformation is identical to the move Transform — the Pokemon copies the target's species, type, moves, ability, stat stages, and all stats except HP. The copy uses 5 PP for each of the copied moves rather than the original PP values.

What makes Imposter competitively devastating is its speed — the transformation happens on switch-in, before any moves are executed, meaning the opponent has no chance to react. This is exclusively Ditto's ability and is the cornerstone of its competitive viability. By copying a boosted sweeper, Ditto can turn an opponent's setup against them, inheriting any Attack, Speed, or other stat boosts they have accumulated. However, Ditto's low base HP is not copied over, which often leaves it frailer than the Pokemon it copies. Imposter also fails if the opponent is behind a Substitute or has used Disguise. Introduced in Generation V as Ditto's Hidden Ability, Imposter is unique to a single Pokemon.