Billy Nolan was the leader of a gang and the boyfriend of Chris Hargensen.
He led a group of his friends to butcher two pigs on a farm, so that their blood would be thrown on Carrie and Tommy at the prom. Billy deliberately did not tell the others what he wanted the blood for, hoping they would get in trouble over it. He then broke into the school by himself and rigged the buckets. He did not really care who the victim of the prank was, as he just wanted to ruin the night for everyone. He actually seemed to hope that Chris herself might be the victim of the prank.
When Jackie Talbot warned him and Chris that the town was burning, Billy rebuffed him and told him to go home. He then intended to dump Chris as soon as possible and make good on his own escape. As soon as he and Chris got into his car, Carrie appeared in front of them. Billy immediately hit the gas pedal and tried to run her over, but Carrie used her power to crash the car, killing him and Chris.