The Clone is Maxwell's rival that steals any holdable objects.
The Clone steals anything apart from holdable animals, flowers, organs and subatomic particles. A notebook clone's behavior is close to Maxwell, as he goes around summoning objects, just like Maxwell. If Maxwell uses a Cloning Machine, it will spawn his clone.
Clone is the old sprite by Scribblenauts Unlimited and similar to Doppelganger.
A sprite comparison between Female Clone and Doppelily in Scribblenauts Unmasked. Even when both are spawned with the "Colossal" adjective as shown in this picture, Female Clone appears taller, while Doppelily is more rounded.