Kai Leng comes off as a whiny child of a character. With no development whatsoever in the game, no clear motive or goals other than being a puppet for the Illusive Man, his characterization is shallow and meaningless. Existing for no other reason than because Bioware wanted an active enemy with a face, he is kept alive again and again due to plot contrivance, not because he is capable, but because the game demanded it. In fact, the game shows Kai Leng as anything but capable, being easily fought against in close quarters by a man who can barely breathe, who had just made his way through the Citadel sounding like he was on his last leg. His need for constant backup during his fights with Shepard and co speak of how poor his skills really are, and only serve to make the game's insistence of his survival all the more contrived.
There's this video log that you can find while in the Illusive Man's base, where Kai Leng is whining, literally whining, at the Illusive Man about how he couldn't kill Shepard. It sums up Kai Lengs character perfectly.
