Kevin is one of many of Gru's minions, and seems to work in the research department of the laboratory.

(NOTE: Officially there are two Kevins in the Despicable Me franchise.  Each is an entirely different type of minion.  This particular Kevin appears only briefly, and is referred to by name, specifically by Gru, during his initial speech about his plan to steal the moon...and then during the scenes where Gru is actually stealing the moon.  Kevin, after having been shrunk--when Gru uses the shrink ray on him to show Mr. Perkins that he in fact has it in his possession--appears a few times in shrunken size, and then returns suddenly to normal size at Dr. Nefario's desk, which alerts the doctor to the dangerous temporary effects of the shrink ray.)