In order to unlock this character, after the third arc of Ultimate Road go to the Chunnin Exam building and challenge Rock Lee to a push up battle in order to learn his Jutsu. After that, go outside the building and talk to Hiashi Hyuuga. He will ask you to tell Hinata that he wants to see her battle Neji. Go to "Trail" and you will see Hinata standing on the bridge. Tell her about the challenge and she will agree to it. Go back into the Chunnin Exam building and you will see Hinata and Neji standing together. Talk to either one and a dialogue will start between Hinata and Neji. When they are done talking, you must fight Neji as Hinata and win with a Level 3 Chakra and when Neji has zero Chakra. When the battle is done, go outside the building and talk to Hiashi again. He will be surprised and you will unlock Byakugan Hinata.