Ditto is a Splixson (based on the word "split") from the Earth-like planet Hathor.
Ditto is a humanoid child alien with the ability to clone himself without limit. Every clone operates independently, regardless of whether or not Ben wants them to, and 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, which negates the usefulness of making large numbers of clones, explaining the reasons why Ben rarely creates many clones of Ditto in the limited times he has appeared. The child-size Ditto is also no stronger nor faster than his size and build suggests.
Ben activated Ditto accidentally in the episode "Divided We Stand", to fight against the seagull mutated by Dr. Animo. His first intentional Ditto transformation was in the same episode, when Ben wanted to have fun in the beach and Ditto can also turn into eighth of them.