Avengers: Who is Ultron?

Ultron is probably the most resilient, ruthless and persistent enemy the Avengers have faced and for a very simple reason: he is not alive.

True. Ultron is a robot who was created by one of the original Avengers, Dr. Hank Pym a.k.a. Giant Man. Ultron was not supposed to be evil but  a mere aid to the Avengers' cause, however, he was created using Pym's  brain patterns so its intelligence evolved and soon Ultron became aware. Because he had human brain patterns, Ultron developed unusual directives and one of them was to consider all mankind as a threat (including his creator) and therefore all humans are his enemies and subject to termination.

Ultron is a machine so he never gets tired and he never quits. Whenever he attacks he has a plan and calculated all the possible outcomes. Every time he has attempted to impose his rule the Avengers have thwarted his efforts and still Ultron has managed to transmit a copy of his consciousness to someone else who was never told he or she was the unsuspected carrier of a "copy of Ultron".

Ultron has upgraded himself several times and his body is every time more intimidating since he used vibranium (the same metal that covers Wolverine´s skeleton).

He has been defeated every time but he always finds a way back, each time more deadly than ever.  

And Ultron is the villain in Avengers 2: Age of Ultron!