You can regain the shield by rotating stances - you'll begin to do so automatically after a certain amount of time spent with the hero.

You can see the exact moment when your guard switches off by looking at the shade of the stance indicator. The shield lasts for a very brief moment of time. Orochi, as well as all other assassin classes found in game, can switch to one of the stances (top, left, right) to receive a temporary shield that block every attack coming from that direction. Other than that, his strikes are really fast - a player not accustomed to fighting against Orochi will have a tough time blocking or parrying his attacks. Orochi has numerous counters at his sleeve, very useful against opponents that attempt to simply push through your defense. Orochi has the best mobility from all classes.