Abel was the brother of Cain.
Although originally thought of as God's favorite, Cain revealed that Abel was in fact talking to Lucifer. In an effort to protect his brother from corruption, Cain offered himself to Lucifer so that Abel would go to Heaven instead. Lucifer agreed, on the condition that Cain himself would send his brother to Heaven. Thus, Abel was struck down by Cain using the First Blade. Abel was taken by Heaven, while his older brother became the most terrible demon to walk the Earth.
Cain would eventually confess the true circumstances of his act of killing Abel to their direct descendant Dean Winchester and the demon Crowley, reflecting once again how much Sam and Dean's relationship not only reflected Michael and Lucifer's but also Cain and Abel's.