Ditto is a playable alien.
Ditto is a small, humanoid alien with the ability to clone himself without limit. Every clone operates independently, regardless of whether or not Ben wants them to, and each clone is indistinguishable from the others. Though the clones are autonomous, they share each other's pain, and the death of one clone will cause the death of the rest (as seen with Dr. Animo's Ditto-enhanced Stinkfly mutants) which negates the usefulness of making large numbers of clones. The child-size Ditto is also no stronger not faster than his size and build suggests.