In Gaiman's Sandman universe, the biblical Cain and Abel come to live in the Dreaming, at Morpheus' invitation. They live as neighbours in two houses, somewhere in the Dreaming. This may be based on the verse in the Bible which says Cain was sent to live in the Land of Nod.
Gaiman's Cain is an aggressive, overbearing character who kills Abel frequently in a kind of macabre form of obsessive-compulsive disorder. In the Dreaming, Abel's death is impermanent, and he seems to recover after a few hours or so. Cain is a thin, long-limbed man with an angular, drawn face, glasses, a tufty beard and hair drawn into two points above his ears. Gaiman's Abel is a nervous, stammering, but kind-hearted man. Abel is quite similar in appearance to Cain, with a tufty beard and hair that comes to points above his ears. He is slightly shorter than Cain, though, and fatter, with a more open face.
Cain seems unable to control his frequent murders of Abel, and in his way expresses remorse for them; there is a genuine bond between the two, despite the surface contempt. Abel remains dedicated to Cain, and frequently dreams of a more harmonious relationship between the two. He is delighted with the pet gargoyle Cain gives him as a gift, and which he first names Irving, but after a quick death renames as Goldie, Cain having forcefully insisted that the names of gargoyles always begin with a "G".
The two frequently play key roles in the story of the series; it is they who take Morpheus in on his return to the Dreaming after his escape from capture, and they aid The Corinthian with the child Daniel during The Kindly Ones, the ninth story arc of the series.
Together, Cain and Abel know stories of mysteries and of secrets; Cain is the teller of stories of mysteries, and Abel the teller of stories of secrets. According to their appearance in Swamp Thing, the difference is that a mystery may be shared, but a secret must be forgotten if one tries to tell it.