Dragged Into Drag: A couple of times, Kenan's plans require a girl, which means Kel has to dress up as one. People can't seem to see through the disguise, either (even when he pretended to be pregnant!).
Only Chris seems to noticed.
G-Rated Drug: Kel's addiction to Orange Soda. There's even an episode dedicated to everyone trying to break his addiction.
Genius Ditz: Kel scored a 98 (out of 100, on this show) on the IQ Test.
Butt Monkey: Tons of examples, but this tropers favorite was in the Christmas Special where he was beat up by a robotic Santa Claus while all the customers sat back and watched. Not knowing it's a robotic dummy.
Character Tics: He twitches whenever there's trouble. Though it was only shown in the pilot episode.