Au Co was a Vietnamese girl killed by Mr. Petrelli on a mission during the Vietnam War. Arthur Petrelli and Daniel Linderman were on a mission to locate an airplane.
They found they were too late and there were no survivors, but "Dallas" decided to carry on with the airplane's mission to destroy Au Co. They presumed that it was the nearby farming village, but later realized that it was a young girl with the ability to make plants grow at a rapid rate when her guardian shouted her name when Dallas started to destroy the village. She was shot to death by Petrelli despite Linderman's best efforts to stop him.
This event though later on became the starting point of Mr. Linderman's and Petrelli's friendship.