Kevin is a half-human, half-Osmosian (alien). though it's unknown which parent was human and which was alien. at first he could only absorb energy and release it at will, but it evolved into letting him absorb the properties of matter that would cover him in a layer of said absorbed matter.
for extra information: http://en.wikipedia.org/wiki/List_of_main_characters_in_Ben_10#Kevin_E._Levin_.2F_Kevin_11
Kevin is the last one on the list.