Niklaus Mikaelson (also known as Klaus) is the fourth born child of Esther,
the stepson of Mikael, the younger half brother of an unnamed child (assumed
to be named Aaron), Elijah and Finn, and the older half brother of Kol, Rebekah
and Henrik. He is an Original/Hybrid who is said to be the most hated and feared
of all the Originals.

During his mortal life, Klaus is described as impulsive, reckless, careless,
adventurous, and a risk-taker; a daredevil at heart. Katherine Pierce ran from
him for over 500 years, as well as Rose and Trevor, and now Klaus is after both
Katherine and Elena, the two Petrova Doppelgangers. The reason why Klaus was
after first Katherine and then Elena was because he needed the Petrova
Doppelgangers in order to break his Hybrid curse. Klaus used Alaric's body to
educate himself on his new enemies (Stefan, Damon, Elena, Bonnie, Jeremy, etc.).

He succeeds in breaking the 1000 year curse that held his dormant werewolf gene
in check, and he now wants to create his own race of hybrids that will make sure
no one will dare challenge him ever again.

After many attempts to kill Klaus, Stefan and Damon succeeded in defeating him
via a desiccation spell by Bonnie through Stefan. Alaric Saltzman killed Niklaus in
the final episode of Season 3 by staking him with a White Oak Stake. Klaus' spirit
was resurrected in Tyler Lockwood's body through a spell conducted by Bonnie
Bennett. The state or status of Klaus' original body is unknown.