Neji is on the ninja team with Rock Lee and Tenten. He is first introduced during the Chunnin exams and demonstrates incredible ability with his Byakugan, which usually plays a major role in his character in the Naruto games. While he is initially portrayed as a villain, Neji quickly comes around to the good side. He fights with a style called 'The Gentle Fist'. He strikes the weak points in a persons chakra (or energy) and shuts them down one by one. In time his opponent simply can not use any of the Ninja's best powers.
Neji, like many others in Naruto, has a tragic past. His father was a twin born second, and thus relegated to a role of subservience. He was a good man, but secretly held a resentment over how for the matter of a few seconds separated the Master of the Clan from a mere leader of a sub-section. His son, Neji, was originally a kind boy. But when politics stepped in the way and only his father's corpse (posing as his uncles to their enemies) would avert war. His father accepted this Fate, but Neji had both a deep resentment towards his Family and Fate in general.
Neji grew up with a lot of secrets. One of which was he could be killed by the main branch at any time for any reason. He hides the tattoo that allows this, as it is imprinted obviously into his forehead. And the purpose of this is to maintain subservience into his branch of the family into the main clan. Neji hated the role he was to play. He hated his uncle and cousins, one of which was Hinata, even though he is sworn to protect them.
He's considered one (If not the very) best of his class, a real rival for Sasuke in terms of genius. Even his uncle is surprised at the depth of his skills when he cuts through all challenges and duels with Naruto with techniques he should not have been able to master.
But before that he had a bout with Hinata. She did not quit, he did not stop teasing her about how weak and suffering she was. It was only when Hinata was near death and she told him how it was actually Neji was the suffering one who had lost his way did he lose control and try to kill her. He was stopped by no less than 4 of the team leading Ninjas present.
Naruto's fight was supposed to be very one sided. He considered a Genius, and Naruto a fool. Neji was an Elite bound to his family's service before birth as Naruto was bound to the Demon Fox. Naruto was motivated to change Neji's view on things once he heard some of the stories of Neji's life. He promised he'd change that clan when he was Hokage. After Naruto beat the overwhelming odds Neji walked away with some new understanding, and a little acceptance that Fate wasn't the all consuming vice that he thought it was. He now believed he could change things, even if just a little.
This pulled him out of his depression and changed his outlook, and the shape of his tattoo.
Neji sacrificed his own life to save Hinata and Naruto, in Chapter 614. He was the first of the "Konoha 11" to die.