Mercedes is the Countess de Morcerf, the wife of Fernand and mother of Albert. In her youth, she fell in love with Edmond Dantes(who would later become the Count) and became engaged to him, but never married when he was accused of treason and thrown in prison, later believed to have died. Even 20 years later she continues to miss and love him deeply. A commoner, she is a kind woman who puts on airs of aristocracy and tries to keep her son from the horrors of the past.
After changing their last name to Morcerf, Mercedes and Fernand lived a life of luxury and wealth, even moving to Paris and having a son named Albert later. But despite her new life, Mercedes felt sad and unsatistified.
She still carries out her strong feelings for Edmond Dantes. When she finds out The Count Of Monte Cristo is Edmond Dantes, she tries to convince him to love her once again, as Edmond Dantes. Even though the Count refuses to have her still think of him as Edmond Dantes (even though he knows he is still Edmond Dantes), Mercedes still loves him more than he thinks.
When Mercedes finds out Mondego was responsible for framing Edmond/the Count 25 years ago, she says her farewell to Mondego, unable to continue to bear his lies and deception, but is shot by him. Though she is able to survive, she is bedridden. When the Count/Edmond dies, she is seen crying in her sleep after being shot by Fernand, sensing that her love was passing on.
Five years after the events of the series, Mercedes moves back to her childhood hometown and is seen standing in front of The Count Of Monte Cristo/Edmond Dantes' and Count de Morcef/Fernand Mondego's graves in Marseilles.