Originally the relationship between the pair was as close as sister's. Tituba was bought by the Walcott family when she was only a child and was treated with kindness especially by Mary.
When Mary discovered she was pregnant with John Alden's baby she helped the young Mary sacrifice the child to Satan turning her mistress into a fellow witch and becoming her most trusted accomplice and ally with her quest to complete the grand rite.
Tituba is shown to care very deeply for Mary and becomes jealous of both John Alden when he returns to Salem and Mercy Lewis. When Tituba is arrested and tortured by Increase Mather in Departures she accuses John Alden of witchcraft inciting Mary's hatred.
In All Fall Down when Mary discovers the child she sacrificed is in fact alive and that Tituba had known this she vows to make her pay.
As a child Tituba was bought by the Walcott family and became very close to their daughter Mary and spoke of the kindness she showed her in their childhood. When Mary discovered she was pregnant with John Alden's baby it was Tituba who helped her sacrifice the child to Satan turning her mistress into a fellow witch.
Throughout The Salem Series
In The Vow after the dinner party, Mary angrily confronts Tituba who was the one that had led her to believe that John Alden had died in the war but Tituba is unrepentant and reminds her who had saved her and gave her all that she desired. The following evening after Giles Corey has been arrested under the accusation that he is a witch Tituba leads Mary away from his cell so that they can begin the witches sabbath but their circle is broken by an unknown source.
In The Stone Child Tituba stumbles upon Mary in her bedroom as she weeps feeding her familiar and reminds her that her tears may sour the milk, while Mary reminisce of what might of been and is further shocked to discover that she is willing to use a monstrous birth as a sign to incriminate Bridget Bishop.
In In Vain Tituba questions Mary in whether she wants to save Isaac Walton because of John but she reassures her that it has nothing to do with John and everything to do with controlling Magistrate Hale.
In Survivors following the discovery that Mercy Lewis is no longer under Mary's control a William Hooke blackmails Mary into giving him access to the ship that is currently docked in Salem or he will reveal she is a witch, so she tasks Tituba with removing the problem while she deals with Mercy.
In Lies after Tituba summons the spirit of William Hooke. She warns Mary that the hive plots behind her back and she must complete the Grand Rite, if she wants to save John and reveals that he is murderer.
In The Red Rose and the Briar following the apparition of Mercy Lewis Tituba advises Mary to kill the girl but is later shocked to learn from The Seer in what Mary is up to in the woods. That night she warns Mercy that Mary is her mistress and soon questions Mary for her reason for turning her into a witch. While Mary explains that she cannot point the finger at them if she is a witch herself.
In Our Own Private America following the death of Rose Mary discovers that the Malum is in the hands of John Alden and informs Tituba of her plans to invade his dreams so that can discover its location, but a worried Tituba is warns her of the dangers that could happen to both of them, but Mary is adamant that she wants Tituba to send her into his subconscious and later reassures her she will succeed. The next night after Mary learns of its location, Tituba warns her to stay out of John's head but unbeknownst to her she continues to invade his dreams and later discovers Mary in danger after she loses control of his subconscious but successfully wakes her.
In Departures with the arrival of Increase Mather Mary decides to send her husband away so that he does not discover that his friend George Sibley is under a spell but the next morning after his departure she wakes with her familiar in her bedroom. With the help of Tituba, they cast a spell transforming Mary's toad familiar into a bird and Tituba reassures her that the bird will lead her to her husband but his whereabouts is also discovered by Increase who returns to Salem with his friend now in his care endangering them both.
In Children Be Afraid after Increase refuses to place George back under his wife's care, Tituba questions if he has spoken yet and reveals that the elders wish to see her. She later warns her of the dangers Mercy poses as she cannot be controlled but Mary believes her to be jealous and cruelly rejects her confession of love. That night a helpless Mary watches as Tituba is arrested by Increase following Mercy's accusation that she is a witch.
In The House Of Pain Mary tries to convince Increase to release Tituba back to her but he refuses and later that night Increase visits her and reveals that she has named Mercy and John as being fellow witches.
In Cat And Mouse Mary reveals to Mercy that Tituba has been pardoned and released from the jailhouse.
In All Fall Down after the elders learn that Mary plans to flee Salem with John. They summon Tituba, who pays a visit to Mary as she is collecting her things at her home where she informs her the child she sacrificed all those years ago in the woods is in fact alive and if she wishes to save her son she must complete the Grand Rite. Mary agree's but not before vowing that she will make her pay for what she has done to her. Later after Mary sacrifices Increase as the thirteenth and finale sacrifice Tituba reunites mother and son.
Mary: I know well my duties. Attend to your own.
Tituba: Your duties and seeing that you fulfil them is my duty, like it or not.