Kai Leng was pretty good in Retribution
(we'll just pretend that other book he also appeared in never happened
). He was a murderer and a bigot and I hated him, but you're clearly not supposed to be rooting for him.
He should have been introduced earlier in ME3, ideally during the Mars mission. He desperately needed more characterization. And his weak boss battles didn't help, either. "I have to recharge! Cover me!"