Niklaus Mikaelson. 

Klaus was the biological son of Ansel and Esther Mikaelson, the step-son of Mikael, and nephew of Dahlia.

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 species of werewolf-vampire hybrids.

In 1492, he finally got the 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 to help him. During his search for a way to break the curse, Klaus hunted down his family and neutralized them. He also hunted Katerina, who had escaped him and stolen his moonstone. However, it should also be noted that he was also on the run from his step-father, Mikael, as Klaus had an abusive childhood.

Besides his family, during the 1920's, Klaus formed a strong bond and brotherly relationship with Stefan. He then met with the vampire, Caroline Forbes, and starts to care about her, eventually falling in love with her and tried to win her love, and after a long time they were friends. In New Orleans, he had a father/son bond relationship with Marcel whom he works with in ruling New Orleans until Klaus discovered his treachery for bringing Mikael to New Orleans. They reformed their friendship when Marcel saved his daughter and they worked together to bring down their enemies

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