Samuel Winchester
Sam had always been different. He was
different than the kids he went to school with
and he was different than his father and
brother. He just didn't know how different.

Neither did his father until Magdelena Hsiao
abducted them, and then gave John the cold
hard facts about Sam.  He was a Nephilim as
she was. A child born of the union of a
possessed man, and a woman.

Sam loved John but always felt that Dean
was more of a parent to him when he was
little than their father. Most of the anger he
felt toward John, was actually for Dean's
sake. Not that Dean ever understood that.
Dean revealed to Sam that he was
like the woman that had tortured him
in their youth and Sam knew what he
had to do. He had to leave before
that happened.

So when they tracked down their half
brother, Jack, Sam took off, and this
time broke off all ties. The yellow
eyed Demon made it plain that
contact with Dean... would kill him.