*The following text may contain spoilers.
Niklaus Mikaelson, often called Nik or Klaus, is an original vampire-werewolf Hybrid born in Mystic Falls around 1000 years ago. He lived with his witch mother Esther and his step-father Mikael. He is the third oldest among the Mikaelson siblings. He has two older half-brothers Elijah and Finn and 3 younger half siblings Kol, Rebekah and Henrik.
Klaus’ biological father was a werewolf. Many years before, his mother had an affair with a man from a different species. So Nik was born with a werewolf gene in him.
Klaus was close to his family with the exception of Mikael. When his brother Henrik was killed by a werewolf their mother was devastated. To protect their children she and Mikael performed a ritual which transformed Klaus, his siblings and his step-father into vampires. They are the first generation of Vampires. The “Originals”.
When he made his first human kill after becoming a vampire, Klaus’ werewolf side was activated, thus Mikael came to know about Esther’s unfaithfulness. In a fit of rage, Mikael hunted down and killed Klaus’ biological father and his entire family igniting a war between the two most dangerous species: vampires and werewolves, that has been perpetuated across the ages.
After discovering his werewolf bloodline, Esther placed a curse on Klaus which forced his werewolf side to become dormant and then she abandoned him. Enraged Klaus murdered Esther and framed Mikael for the kill thus lying to all his siblings.
After the death of their mother, Kol and Finn fled leaving Elijah, Klaus and Rebekah who vowed to each other that they will stick together as one always and forever.
Klaus shut his humanity off to anyone but his family, killing everyone who crossed his path. Venegeful and power-hungry, he stopped at nothing to achieve his ends and remove those who stood in his way. Klaus had no tolerance for those who betrayed him or abandoned him. He daggered each and every one of his siblings, starting with Finn and Kol. Then in 1920 he stabbed his sister when she chose to go with Stefan and leave him alone. He undaggared her in 2011 when Gloria, a witch, needed something from Rebekah.
He spent most of his life trying to break the curse Esther placed on him in order to create hybrids of his type. To break it he required a moonstone, a werewolf, a vampire, a witch and a doppelganger to sacrifice and after 1000 years, he succeeded, using Elena (as a doppelganer), Jenna (a vampire) and Jules (a werewolf).
In order to create his hybrids he needed the doppelganger’s blood. He was successful in creating them by killing werewolves and feeding them with Elena’s blood. Tyler Lockwood was his first hybrid.
Soon after that Rebekah learned that Klaus had killed their mother. In an attempt of revenge on her brother she called Mikael who had means to kill Klaus. In a fight gone awry Klaus grabbed the upper hand and killed Mikael with a stake meant to kill him thus ending their centuries old vengeance and running.
Among his siblings he is closest to Rebekah and is very protective of her. He never left her in 1000 years.
As a hybrid, Klaus can’t be killed with any usual methods that kill vampires, werewolves or even originals.