Nero was a Romulan mining captain from the 24th century whose journey into the past created the Kelvin timeline and led to the destruction of Vulcan.
Nero commanded the Romulan mining ship Narada. When Romulus was destroyed by a supernova, he blamed Spock, who had tried and failed to stop the disaster, and the Federation for letting his world and his wife die.
Nero's ship and Spock's vessel were pulled through a black hole created by red matter and sent back in time. The Narada emerged first, in 2233, and attacked the USS Kelvin, killing George Kirk on the day his son James was born and changing the course of history. Nero then waited years for Spock to arrive. When he did, Nero stranded him and later used red matter to destroy Vulcan, killing billions. He next attacked Earth, but James T. Kirk and the younger Spock boarded the Narada, rescued Captain Pike and stopped the attack. Nero refused help and died as his ship was destroyed.
Nero's revenge reshaped the lives of Kirk, Spock and the whole Enterprise crew, and his attack is the event that separates the Kelvin timeline from the prime one.