Eddie Dean is called The Prisoner because he is a prisoner of heroin addiction.
Eddie is introduced in Stephen King’s The Drawing of the Three, the second novel in The Dark Tower series. Roland Deschain draws him through a magical door from 1980s New York, where Eddie is acting as a courier for a cocaine-smuggling operation. Eddie’s drug dependence, particularly his heroin addiction, is central to his introduction and to the prophecy’s label.
The title The Prisoner refers not only to Eddie’s physical circumstances but also to the addiction that controls his choices and leaves him psychologically trapped. Roland initially sees him as a troubled and unreliable young man, but Eddie gradually confronts withdrawal, grief, fear, and the influence of his abusive older brother, Henry. Through the series, he becomes a skilled and committed member of Roland’s ka-tet.
The prophecy’s other labels identify Roland’s future companions through symbolic descriptions. Susannah is associated with The Lady, while Jake Chambers is The Boy. Eddie’s designation is specifically tied to addiction, not gambling, the American justice system, or simply a tortured state of mind.