In the story of Death Note, the ultimate battle is between Kira and the mysterious L, both of whom claim to be true justice. But there is more than one Kira and more than one L.
The original L was the man known as L Lawliet, but he lost his life in the pursuit of Kira the mass murderer. Light Yagami, Kira, was sure that the coast was clear, but the L moniker had an heir, and that heir was eager for revenge. Enter Near, the child prodigy.
Near, also known as Nate River, grew up in Wammy's House, an orphanage in England dedicated to raising the smartest boys and girls around. These students, as Aizawa and Matsuda later found out, are given a rigorous education and tested constantly to narrow down who might become the next L. This also was the home of the original L, and everyone there, especially Nate River and Mihael Keehl, looked up to him with awe and respect. But late in 2004 (in the manga), Roger, Wammy's House's director, received the grim news of L's demise.
So, Roger summoned Nate and Mihael, who now went by Near and Mello, to his office to share this news. L had never decided on an heir, so both boys were offered the position jointly. Near, being a practical and logical person, agreed to the proposition, but he was soon left with the title alone, since Mello's competitive nature drove him to go and find his own path. Accepting this change of events without complaint or comment, Near practiced to become the next L, but first, he had to deal with Kira. And someone else was claiming to be L.
In 2009, Near gained the American president's trust and cooperation and was allowed to lead the Special Provision for Kira, a well-funded and well-staffed agency operating in New York City. Near began cooperating with the impostor L, despite his misgivings about this L's apparent capabilities, and he pragmatically took L's side to confront Mello when the latter launched his hostage scheme. Near was dismayed that L allowed Mello to get the notebook in exchange for Sayu Yagami's life, and Near watched curiously as the impostor L and Kira cooperated to get the notebook back from Mello. By now, through patient observation and analysis, Near suspected that Kira and the fake L were the same person.
Near was put on the defensive when a Kira crowd, led by Demegawa, stormed his New York City HQ, so he fled with his allies to a new, more secret base. Near crossed paths with Mello soon after that, and the old rivals agreed to race to find Kira, and an unspoken ceasefire was called. Believing that "L" was Kira, Near switched gears and began to pressure L more and more, using his own agents, Mello and the Japanese police to try and find the truth. Near also chased after Teru Mikami the prosecuting attorney and Kiyomi Takada, seeing them as Kira allies, and he prepared for a final confrontation with with the murderer.
In January 2010, Near met with "L" and his group in a seaside warehouse, and Near trapped Light Yagami with his fake notebook swap. Light exposed himself as Kira, and he and Teru were cornered. Near listened patiently as Light tried to defend himself as Kira, but rejected every word of it. To Near, Kira is a mass murderer, period, and it's personal because Kira slayed L Lawliet. What's more, Near paid tribute to the fallen Mello by crediting his role in the investigation, showing he isn't too petty to let his feud with Mello get in the way of true justice. Near watched as Ryuk turned on Light and killed him, and Near was free to become the new L. As of 2011, Near was comfortable in his role, and he had the Japanese police working closely with him.
Near is one of the anti-Kira big three, along with Mello and L Lawliet. Among them, he has the highest IQ (according to official sourcebooks), but he has the lowest initiative. Basically, Near is the archetypal brainy type that relies heavily on others to carry out his plans. Near has little physical strength or training, setting him apart from the gun-toting Mello and the shockingly nimble L. Instead, Near is cautious and patient, willing to outlast his enemies rather than take them out with daring strikes like L's enrollment at To-Oh or Mello's action heists.
For example, Near had only a peripheral role in the hostage incident, watching "L" make the exchange and then watching again as he raided Mello's hideout to get the notebook back. Instead of taking the fight to his enemies, Near allowed Light to give himself away as Kira, since the cooperation between L and Kira was all too convenient in more ways than one. Although Near promised Mello that he'd race him to find Kira, he actually allowed Mello to makes the most moves, and he turned the Japanese police against Light and Misa Amane.
Near has also demonstrated a "ends justify the means" attitude that echoes Mello's, but in a less flashy manner. He rashly captured Mogi and Misa, and made no apology. He's even willing to lie, often to Light, to trick and disorient him.