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.