Mickey
One of the titular "blood brothers," Mickey is one of two twins separated at birth. At a young age, he develops a close bond with his biological brother, Eddie. Mickey is confident and tough, as he is forced to be resourceful in dealing with his cruel older brother, Sammy. As an adult, he eventually develops romantic feelings for his friend Linda and marries her. His life takes a dark turn after he loses his job and does time in prison for taking part in a robbery.
Eddie
Eddie is Mickey's twin. He is given up by Mrs. Johnstone to Mrs. Lyons. Growing up in the Lyons' household, he is given financial stability and better educational and job opportunities. He meets Mickey one day on the street and the two instantly become friends. Their lives diverge significantly after he leaves to go to college and later becomes a local politician. He also has feelings for Linda, but does not seem to act on them. He is portrayed as a sweet and thoughtful young man who is often thinking about how he can help others.
Mrs. Johnstone
Mrs. Johnstone is the mother of Eddie and Mickey. She lives in Liverpool and works as a maid. She supports several children on a single salary. She has twins at the beginning of the play and gives one of them away to her employer, Mrs. Lyons, who is wealthy and unable to have children of her own. Mrs. Johnstone is hurt by this loss and feels a bond with Eddie throughout his whole life. She is a loving but firm mother.
Mrs. Lyons
Mrs. Lyons is a wealthy woman who adopts Eddie as a part of a deal with Mrs. Johnstone. She quickly becomes bitter and hateful, as she thinks Eddie likes Mrs. Johnstone more than her. Her mental health begins to deteriorate and she refers repeatedly to various superstitions about twins separated at birth. She becomes so vengeful that she tries to kill Mrs. Johnstone with a kitchen knife and later instigates Mickey's killing of Eddie.
Linda
Linda is a childhood friend of both Mickey and Eddie. She gets into a relationship with Mickey despite Eddie being more reliable and stable. They get married after she gets pregnant. She does her best to comfort Mickey when he gets out of prison but has little success. Mickey's jealousy about her close relationship with Eddie sets off the play's tragic end.
Sammy
Sammy is one of Mickey's older brothers. He is involved in violence and crime from a young age. He later murders someone during a robbery and gets sent to prison. He is depicted as being cruel and selfish.
Narrator
The narrator recounts the events of the story and introduces some of the group songs. At the end of the play, he questions how this turn of events came about.
Mr. Lyons
Mr. Lyons is Mrs. Lyons' husband. He is frequently away at work and does not play an active role in raising Eddie.
Policeman
The local policeman frequently scolds both Sammy and Mickey. His warnings about Sammy to Mrs. Johnstone cause them to move.
Miss Jones
Miss Jones is Mr. Lyons' secretary. She writes memos for him and is featured in the song "Take a Letter, Miss Jones," where she writes people's termination letters.
