Nero

Star Trek
Nero was a Romulan miner originating from the late 24th century, and captain of the mining vessel Narada. Following the destruction of Romulus in 2387, Nero sought vengeance against those he felt were responsible, ultimately resulting in his being transported back in time to 2233.

Nero's actions in the past resulted in the creation of an alternate reality. In this reality, Nero was responsible for the destruction of the USS Kelvin as well as the death of its senior officer, Captain Richard Robau. George Kirk's kamikaze strike against Nero's ship, as a result of Nero's kill-or-be-killed dilemma, altered the upbringing of his son, James T. Kirk, who in this timeline did not join Starfleet until 2255.

Nero was later responsible for the destruction of the alternate reality's Vulcan, which resulted in the deaths of Amanda Grayson and the majority of the Vulcan race. Nero also attempted to destroy this timeline's Earth, but his plot was foiled by Jim Kirk and the crew of the USS Enterprise.

Ultimate Marvel
Nero and his colleague Si Ma were scientists who worked for Bio-Armor, a technological research firm contracted by the Stark Defense Corporation. Under the leadership of SDC's founder Howard Stark, Nero and Si Ma were instrumental in creating a bacterial body sheathe that would render an individual impervious to most conventional forms of injury. Nero often volunteered to test the bio-armor's capabilities. Howard Stark lost controlling interest in Stark Defense Corporation to his rival Zebediah Stane, but fortunately, he had no legal ownership over projects designed by Bio-Armor.

Frustrated beyond measure, Zebediah Stane was determined to learn the secrets behind Stark's armor coating. He sent some men to track down and abduct Nero, bringing him back to an undisclosed facility. Stane applied one of his own inventions, a mind-reading skullcap called the Lid, to Nero's head. The machine violently ripped all information concerning the bio-armor's designs from Nero's mind, leaving him insensate. Nero survived the experience, but his intellect was reduced to that of a toddler.

Stane still had some mental control over Nero however. When he learned that Howard Stark used the bio-armor to save the life of his son Antonio, he tried to mind-control the addle-brained Nero into abducting the child for him. Nero resisted the mental summons long enough to scribble out a warning to Howard Stark. To prevent Stane from gaining complete control over him, Nero killed himself by stabbing himself in the ear with a pencil until it pierced his brain.