Keven Ethan Levin started off as a pity thief. In the Ben 10 show, Kevin first met Ben when he was 11 and Ben
was 10. They started off as good friends, but when Ben told Kevin his secret Kevin wanted to use it to get rich and famous this made their friendship go south. When Kevin absorbed Heatblast he turned into a mini version of the hero, Ben eventually stopped him, but he still had fun using him. When Kevin found out that he had all of Ben's aliens he used them to not only get rich, but to also frame Ben. When Ben made fun of Kevin, Kevin turned into a mixture of all 10 original aliens, making him into the ultimate Ben 10 villain. This caused a problem for Ben, however, also an advantage. With Kevin's great power it also came with a disadvantage. Kevin had weaker power of all the aliens, since they were a mixture. So Ben threw him hurling into the sea, thinking that they will not meet each other again. In Alien Force, Kevin joins Ben to help defeat a alien race known as the hybrids. Kevin also formed out of being the mixture of Ben's original 10 aliens. Kevin has a love interest in Ben's cousin Gwen Tennyson
Kevin is a Osmosian. A Osmosian has the power to absorb any kind of material, Kevin got this power from his father. Kevin's father is a Osmosian and his mother is a human, so Kevin is a mixture of a Osmosian and a human. In alien force, Kevin got the power to shape shift his body into various shapes and weapons. This only happened when Ben tried to hack the Omnitrix, at the end of season 3 of alien force, Ben destroyed the omnitrix and Kevin reverted back to his normal form. And regained his regular powers.