Ditto (voiced by Rob Paulsen) is a Splixson (based on the word "split") from the Earth-like Planet Hathor (the first syllable of this name being similar to the word "half"). Ditto first appears in "Divided We Stand".
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 nor faster than his size and build suggests.
Splixians are also very constructive and can work well with one another.
When the Omnitrix times out, the Ditto clones are forcibly recombined into one before the transformation reverts. If one or more clones are isolated from each other when the Omnitrix times out, the timer will reset periodically until they are reunited. This only seems to apply when a clone is physically unable to reunite with the others; in open air, the clones will float to each other over great distances in order to facilitate the transformation. It is implied that the reset "glitch" forces the Omnitrix into a state where the next transformation is timed out much faster than usual, due to the strain of being active through multiple timeouts.
The planet Hathor is home to a race called Splixsons(Simaler to "Split") who have not grown out of Mitosis (that is how they can divide without limit), but they don't fight or start wars (that was made clear in "Divided We Stand") they are more peaceful than other races, they are more like farmers or artists.