They have known each other since childhood and when Mary married George Sibley she took pity on him. Isaac commented in The Vow that Mary has been the only one to ever do him a good turn.
Isaac is very loyal to Mary because of the kindness she has shown him and she has shown to genuinely care for him. In Ashes Ashes Mary vowed to protect him. They are portrayed by Iddo Goldberg and Janet Montgomery.
Both were born and raised in Salem, Massachusetts and are the first borns in the new world. They knew each other as children and during the events of 1685, Mary watched as Isaac was publicly branded. After she became a witch and married George Sibley, Mary took pity on Isaac when no other would.
In In Vain the Magistrate discover's from The Seer that it was Isaac that had broke the witches circle but he was not alone. He captures Isaac without Mary's knowledge with the intent to try force him to divulge who was his accomplice. They take him deep into the woods where. the Magistrate and a fellow witch cast a spell on him but he refuses to tell them who was with him that night so they send him back to Salem after they plant evidence on him that will indicate him of being a witch. A confused Isaac goes to The Divining Rod and attacks the prostitutes and is later arrested by Magistrate Hale, wanting to pacify the magistrate so that she can spare Isaac, Mary creates an illusion of his former lover Abigail Cooke so that she can interrogate him into divulging who really was with him that night and is shocked to learn it was John Alden.
In Departures with the arrival of Increase Mather Mary enlist Isaac's help to take George Sibley to Boston Hospital so that he will not discover that her husband is under a spell but on the way the carriage crashes and George is injured. Isaac returns to Salem for help but John tells him to keep quiet and let him die, but he is coerced into revealing the truth to Increase who goes to save his old friend.
In Children Be Afraid with George under Increase care. Mary gives Isaac a potion for pain to give to her husband but it is really a potion to silence him but he is discovered by Increase who threatens him and he is forced to reveal that it was Mary who gave him the potion.
In Cat And Mouse following John's arrest for witchcraft Mary devices a plan to kill Increase. She summons Isaac to her home and drugs and compels him to lure Increase deep into the woods under the pretence that he had followed and found Mercy there but he is injured during the battle between the two and is brought back to Salem. That night Mary is informed of Isaac's injuries and visits him and at his bedside promises an unconscious Isaac that she will always protect him.
In Ashes Ashes following John's sentence Isaac visits Mary and convinces her to save the love of her life.
In All Fall Down Mary tasks Isaac to take the Malum to the woods and place it in the witches tree and leave Salem. She gives him a pouch of gold so that he can start a new life but instead he takes the Malum to the crag so that he can bury it with the dead but is infected by the red plague it contains.
Mary: Perhaps there is solace to be found in remembrance of times when we've both shouldered worse. Who could have imagined that a single night could have consequences for so many? Not all scars can be seen, but they exist nonetheless.
Isaac: You're Mary. You're Mary. They all call you Mary Sibley. Do you know what I call you in my head, to myself, in my heart? Magic Mary. You were always magic, even back when we was just sprouts and you could do anything, always could. You just have to want it bad enough.