Who is Marley in A Christmas Carol?

Marley, Scrooge's former partner, is the instrument of Scrooge's redemption.

Jacob Marley has died deep in the sin of indifference to the needs of others. It is a fate that is illustrated as one of aimless wandering, heavily burdened with the chains "he forged in life, link by link" through his callous, stingy, materialistic behavior.  

Marley plays a central role in convincing Scrooge that he must change his life or face an eternity of suffering.  It is Marley who visits Scrooge as a ghost and acts as his conscience, reminding him of all the indifference they, as business partners, showed in their efforts to grow wealthy.

Marley must convince Scrooge that there are consequences to his behavior.  He tells Scrooge to expect three spirits.