Andy is another one of the Special Children like Sam.
Andy's mother Holly Beckett was pregnant with him and his twin brother at the age of 18 but gave them up and they both were adopted out to different families. Andy's adopted mother was murdered by the demon above Andy's crib when he was six months old, but Webber's adopted mother wasn't.
Andy is a psychic child like Sam thus gained his powers through Azazel dripping his blood into Andy's mouth as a baby. Andy can control peoples mind,he convinced Dean to give him the impala, and later developed his powers to send psychic images to others.
After Sam Winchester has a vision, Ash helps him track down where it took place, and finds Andy, whose mother was also killed in a nursery fire. Andy uses his power to get Dean to give him the Impala, and later to get Dean to speak truthfully about what he and Sam are doing --- although it is proven that his power has no effect on Sam.
Sam and Dean suspect that Andy has killed people using his psychic powers, but when they search Andy's van they find nothing to suggest that he is the killer. When a woman is killed when Sam is with Andy, and she turns out to be Andy's birth mother they realize that they have the wrong suspect. Further investigation reveals Andy has a twin, Ansem.
Ansem Weems has tracked Andy down, and has been killing people important to Andy. He is about to kill Tracey when Sam and Andy confront him. Just as Ansem is about to make Dean shoot himself, Andy kills him.
Andy is one of the kids bought to Cold Oak, South Dakota by the Yellow-Eyed Demon. Andy reveals he has developed his powers so he can beam images into people's minds, including transmitting gay porn into the mind of a guy he didn't like. Andy is able to transmit images to alert Dean to their whereabouts. He is killed by a demon summoned by Ava.
(Info taken from http://supernatural.wikia.com/wiki/Andrew_Gallagher )
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