Who in Julius Caesar is an example of “Deceit wears the costume of innocence.”

Virtually all of Caesar's assassins could be described in this way; they act friendly to Caesar's face when all the while they're plotting to have him murdered. But one would have to say that Brutus most clearly fits the description. He, more than anyone else, appears to be completely innocent; he's the last man on earth that Caesar would expect to betray him. This is demonstrated clearly by Caesar's famous last words:

Et tu, Bruté?—Then fall, Caesar (III.i).

"You too, Brutus?" Caesar can't believe that his dearest friend, someone he genuinely loved, could be capable of such base treachery. Yet for all that, Brutus still loves Caesar; it's just that, as he explains in his speech to the Roman people, he loves Rome more. Brutus genuinely believes that Caesar wants to make himself king, overturning centuries of republican tradition. If Caesar's allowed to get his way, thinks Brutus, then the Roman people will become little better than slaves, lorded over by a tyrant.
But deceit is still deceit, irrespective of whatever noble motivations there may be involved. And there can be no doubt that Brutus has deceived his friend. Instead of taking a brave, public stand against Caesar and what he sees as his incipient dictatorship, he prefers to hide his deceit behind a cloak of friendly innocence before literally plunging the knife in.