It should be possible to immediately distinguish any two characters in the same game, double so if they are main characters. Dante and Virgil were easily distinguished in DMC3 by the different colors of their coats, their different hair, their different weapons, and also by their vastly different body language. A relatively simple change to Nero's character design could similarly eliminate any confusion between him and Dante. For example, changing his hair color by a little bit, or by completely removing the red in his outfit.
Anyways, I do like what I am seeing so far in the trailers gameplay wise. The ability to change styles mid-battle resolves a major annoyance from DMC3. Also, I am impressed than Nero does fight very differently from Dante, thanks to the special powers of his right arm. It defeats the point of having multiple characters if they fight too much like each other. That was a major problem in Megaman X7 with Axel and X. I am also hoping for a lot of unlockable secret characters. It was a real disappointment that Virgil was not unlockable in the original version of DMC3. Unlockable characters are a king of bonus feature that never grows old.