The Zero on the battlefield is not the real Zero. I was wondering why he simply did not try to manipulate or even Geass the Tian Zi to order Xing Ke to stop attacking after its power difference was evident instead of engaging in a head on battle, but after seeing him at school it's clear. He would've chosen the Geass route if he were able to. As mentioned earlier, it only kept showing the right side of his face through the mask, which is obviously a hint he can't use Geass. Zero on the battlefield is someone in his outfit with a speaker most likely.