Kevin Ethan Levin is an Osmosian and a member of Ben's Team. When Kevin first appeared in the original series, he quickly became one of Ben's most notable enemies. However, he reformed in Alien Force, and became one of the main characters until his departure, along with Gwen in The More Things Change: Part 1.
In the original series, Kevin is a thin and scrawny boy with shoulder-length black hair, pale skin and dark brown eyes with black-discolored marks around them (a sign that an Osmosian has recently absorbed energy). He wore a torn black t-shirt, black studded arm bands, brown cargo pants, black knee-length shoes, and a padlock necklace which is a spoil from his first crime: stealing a bike and using it to run away to New York. After absorbing an Omnitrix feedback, Kevin was able to transform into exact copies of all of the unlocked aliens, missing an Omnitrix symbol. After the energy overwhelms Kevin, he mutated into Kevin 11, a random mixture of body parts from the 10 said aliens which completely destroyed his clothes, except for his cargo pants.
In Alien Force, Kevin grew into a tall and muscular young man. His eyes now lack the black marks (as his body fully recovered from and stopped absorbing energy). His normal attire is a tight black T-shirt over a grey long sleeve shirt, blue pants, and black combat shoes. His hair, although still long, is styled into a choppy mullet. Occasionally, he is seen wearing a short sleeved light blue oil stained mechanic's jacket over a white muscular shirt, blue pants, and his black combat shoes. After he goes through a second mutation, an unbalanced mixture of various matters, the transformation completely destroys his clothes, except his briefs. As a result, Kevin had to wear an ID Mask in public to appear human. In his second mutation he looked like a mixture of solid materials such as stone, wood, Taydenite, and metal. During Kevin's time as Ultimate Kevin, his appearance is again an unbalanced mixture of various aliens that were currently accessible to Ben through his Ultimatrix.
In Omniverse, Kevin wears the same black shirt but the sleeves and collar are roughly cut, the sleeves on his long sleeved grey shirt are slightly torn and he wears blue wholesters. His hair is longer and straighter. His pants have a small tear below the left knee. He also wears a padlock necklace similar to the one he wore when he was 11 but marked with an "11". He also sports black discolored marks around his eyes, making him appear tired; these marks bear similarity to when he was 11 and still absorbing energy.
Later in the series, Kevin grows a few chin whiskers and his grey shirt gains stripes. In Weapon XI: Part 2, Kevin gets the same cross-shaped scar to his chin that future Kevin has.