Niklaus Mikaelson (a.k.a. Nik or Klaus) is the Original Vampire/Werewolf Hybrid, and the son of an unnamed werewolf and Esther. He has six half-siblings: a deceased unnamed older half-brother, four other half-brothers named Elijah, Finn, Kol, and Henrik, and a younger half-sister named Rebekah. Klaus is also the step-son of Mikael, with whom he had a strained relationship before the latter got killed. Little is known about his werewolf side of the family due to Mikael's onslaught.
He is first mentioned in a conversation between Rose and Stefan when she warns him about the Originals. She tells him that Klaus is believed to be the most hated and feared of all the Originals and those who fear him are desperate for his approval. Over the centuries, Klaus has become a recluse who confides in and trusts only the few members of his inner circle.

For over a thousand years, Klaus had been trying to break a curse placed on him. He created the myth of the sun and moon curse in order to find the moonstone and the Petrova Doppelgänger, both of which were needed to break the actual curse. His goal was to liberate his werewolf side so he could sire his own super race of vampire-werewolf hybrids.

In 1492, he finally got his chance to break his curse when he met Katerina Petrova, the final piece necessary to break the curse. However, Katerina managed to foil his plan to use her in the sacrifice by turning into a vampire. For the next 500 years, Klaus searched for a way to break the curse without the doppelgänger, forcing generations of witches and warlocks to assist him. During his search for a way to break the curse, Klaus hunted down his family and neutralized them. He also hunted Katerina Petrova, who had escaped him and stolen his moonstone.