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Upon using Transform, the user becomes the same species as its target, copying all base stats, all stat changes, and the target's exact moveset, each copied move receives only 5 PP, with a maximum of 5 PP each. It does not copy the target's base HP, experience, level, or any status ailments. The user also retains its item. The user can no longer change formes if it would have the ability to do so. Transform cannot be blocked by Protect or Detect, but can be avoided through the "invulnerable state" of moves such as Fly and Bounce. It will also fail if used on a Substitute or on a Pokemon with an unbroken Illusion. If Transform is used on Giratina-O, the user will transform into Giratina-O regardless of whether it is holding a Griseous Orb or not. Similarly, if it is used on Arceus, it will change to whatever forme that Arceus happens to be in, but its Judgment attack will be the same type as the held Plate.
Transform is terribly gimmicky, but has a bit of competitive merit on Mew in UU. A Transform Mew can adapt each match to copy one of your opponent's Pokemon and give the team extra support for that specific match up. However, it's no where near as effective as Mew's standard sets; only use it if you're up for a few losses with laughs along the way.