He says, simply:
Sharlto Copley… is Chappie.
And there’s a new profile for the character created too, though all that says, for now, is:
The character of Chappie is a robot, previously described by Blomkamp as “ridiculous.” He’s also said that the film will take place in an “unusual setting.”
I expect it’s most likely that Copley will play the role through motion capture, though there might some kind of makeup involved instead. Or perhaps Chappie, despite being artificial, will just look entirely human.
The film will go into production this Autumn. In the meantime, Blomkamp’s Elysium is set for release in August. Despite being cybernetically enhanced, Copley is playing a human in that one. A nasty, immoral piece of work.