Ben 10 is not Japanese animation! It is a cartoon produced in the US.
Ben 10 tells the fantastic story of Benjamin Tennyson, a boy who is on vacation with his cousin Gwen Tennyson and his grandfather Max. At the start of their summer trip, Ben is almost hit by a strange alien device. He finds out that it's an Omnitrix, which is a watch-like alien device capable of transforming anyone who possesses it into 10 different alien species, each with their own respective power/ability. From here Ben faces the responsibility of using his powers to fight evil, though Ben is still young and not very mature. However, he'll have to learn to master his abilities to fight off the onslaught of Vilgax, an evil Galactic conqueror who wants the Omnitrix back in order to begin his invasion and conquest of Earth. Does Ben have what it takes to save the world? (Source: Absolute Anime)