Transform is a Battle Skill that is used by Doppelgangers. If used against a player or AI, the Doppelganger will turn into a clonus, with the same stats as the adventurer. Similar to the Roboknight's Copy skill.
Note: Contrary to a real Clonus, a Doppelganger will not automatically counter if targeted with strike.
There is reportedly a glitch where if an adventurer enters a battle against a Doppelganger, and it uses Transform, but a Darkling kills the adventurer, the Doppelganger will stay transformed but as the Darkling (needs verification)