Susannah Dean (also referred to as Odetta Holmes and Detta Walker) is a fictional character from Stephen King's The Dark Tower series. She is introduced in The Drawing of the Three and continues as a main character for the remainder of the story.

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In New York SPOILER ALERT!!!!!!!!!!!!!!!!!!!!!

Odetta Holmes is the only child of Dan and Alice Holmes, a wealthy black couple, living in New York City.
At the age of five, the serial killer Jack Mort dropped a brick from a high balcony on Odetta's head, placing her in a coma. Out of this traumatizing incident, Odetta's damaged mind created a completely separate, second personality named Detta Walker.
The disparity between Odetta's two personalities approaches the level of polar opposites. Odetta is a morally righteous intellectual with a significant level of education and class who participates in civil-rights protests. Detta is a violent, base individual ruled by sexual desire and fueled by hatred towards the white people Odetta passively resists. The two personalities are completely unaware of each other's existence.
In an ironic twist of fate, Jack Mort (presumably, though never explicitly stated, unaware of the coincidence) attempts to kill Odetta again twenty years later by pushing her in front of the A-train at Christopher Street station. At the last second a man pulls her out of the way, though her legs are hit by the train and become separated, restricting her to a wheelchair for the rest of her life.
It is at this point that the Detta Walker personality begins to assert more control over their shared mind. For the three years following the loss of her legs Odetta experiences periods of blank space which her intellect fills in with fictional events. Detta, too, suffers the same effects when not in control and realizes periods of time are missing, though she chooses to ignore this in favor of following her desires. This behavior persists until Roland Deschain "draws" Odetta/Detta into Mid-World from New York City in 1964.

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In Mid-World

When Roland Deschain enters the door to 1964 and drags her into Mid-World, Detta Walker is in control and responds in a hostile manner, calling him a "honky mahfah". Eventually, Odetta Holmes's personality wakes up and tries to dismiss the whole incident as a dream, but eventually realizes the reality of the situation and becomes depressed and hysterical. She eventually finds friendship and love in Eddie Dean, another person drawn from her world; however, the relationship is restricted by Detta's personality.
Detta takes over and escapes when Roland travels through the third door into the mind of Jack Mort. Eddie is left alone in the darkness and paranoid of Detta, who intends to murder him and Roland. She eventually holds Eddie up with Roland's gun and ties him up in ropes before leaving him on the beach to get eaten by lobstrosities.
Roland (in Mort's body in Detta's world) realizes a way to save Odetta's life, and brings Mort down to a subway station. He then calls for both Odetta and Detta to look into the doorway out of Mort's eyes. When Detta and Odetta both look out the doorway, they see each other for the first time and both have combined memories of Mort dropping the brick on their head and then being pushed in front of the train. This has the effect of blending the personalities together, creating Susannah Dean (Susannah because it was the middle name of both Detta and Odetta, and Dean because she is married to Eddie by ka).
Throughout the rest of the series, Susannah remains Eddie's lifelong love and one of Roland's faithful companions.

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In New York again

At the end of the last novel, The Dark Tower, Patrick Danville draws Susannah a door to "happiness" and she travels through it. She ends up in a version of New York, meeting what appears to be Eddie Dean and Jake Chambers (now as brothers and with the last name of "Toren"). Eddie confides in her that he has had dreams of her and believes that he is falling in love with her.
She is the only member of Roland's ka-tet not to have died at some point throughout the series.

END OF SPOILERS