Sister Mary Cynthia (formally known as Cynthia Miller) is a resident of Nonnatus House and a nurse and midwife. She is portrayed by Bryony Hannah.
Cynthia is already resident at Nonnatus House when Nurse Jennifer Lee arrives. Caring, stoical, and intelligent, she finds her midwifery work both challenging and rewarding, a highly desirable alternative to the dull domesticity that would otherwise be her only option. Quiet and sensitive, and also in her twenties, Cynthia becomes Jenny’s confidante and life-long friend.
Cynthia is shown to be the most compassionate and soft spoken of the midwives. She feels things deeply, and has very emotional reactions to a number of her cases. Despite this, and the fact that she is quiet and the 'shy' midwife, Cynthia is very defiant and strict when need be, as shown when she defends a wife from her bully of a husband.
Little is known about Cynthia's family life, but she does mention that her mother made her and her brother's baby clothes, using the mother-of-pearl buttons from her wedding dress. In the start of Series 5, Sister Mary Cynthia confides in Sister Winifred that her younger brother was born with water on the brain (hydrocephalus), causing apparent deformity. The Miller family loved their baby anyway, and Sister Mary Cynthia's mother told her that when people stare at her little brother, stare back and shame them. Sister Mary Cynthia says she never could, because for just a second, she could see her brother through strangers' eyes. He died long before the start of the series.
In the 2014 Christmas Special, Cynthia makes the decision to become a nun and goes off to the Mother House in Chichester as a postulant, despite Trixie's uncertainty about the notion. She returns to Nonnatus House during the course of Series Four, now going by the religious name Sister Mary Cynthia.
Returning in Series Four, Sister Mary Cynthia has moved on from her first six months as a postulant, but still has not undergone her final vows to be considered a full nun. Her life is more focused around being a nun and less about spending time with her friends, often making Trixie sad for how distant they now are. She and Trixie remain friends, with Sister Mary Cynthia being the one who convinces Trixie to finally get help for her alcoholism.
In Series Five, the East End of Poplar was terrorized by a man in the night who attacked women on the streets. His first two victims, a prostitute and a young mother on a late night walk with her infant, refused to come forward to press the police to find their attacker. The prostitute was a mother, who was abandoned by her husband and forced into prostitution to support her children, and she refused to speak up out of fear of her children being taken away. The second victim was on a late night walk with her baby because her child wouldn't stop crying and she was told by a nun at Nonnatus that late night walks might calm him down. Out of exhaustion and frustration, she left her child on the street in his pram and walked around the corner for some quiet when she was attacked. She refused to speak up out of shame.
After attending to a mother and baby at night, Sister Mary Cynthia rode her bike home, but stopped along the pier to pray, as she was missing the nuns' early morning prayers. She was then attacked by the man, and was knocked unconscious. She awoke later on the pier, curled up against a pile of rope, her habit lying a few feet away.
After going home to Nonnatus, she was asked by her sisters and fellow nurses what happened. An angry and upset Sister Mary Cynthia bluntly tells them she wasn't raped, despite her battered appearance and the bite marks on her neck. She violently flinches back when they attempt to touch her and goes to the bathroom for a scalding hot bath, despite Trixie's pleads. Sister Monica Joan attends to her in the bathroom, washing her down and holding her gently as her Sister cried. Sister Mary Cynthia expresses to her fellow nuns that she is angry that God allowed this to happen to her, as she has pledged her life to Him and stopped to pray to Him before she was attacked. To make matters worse, she says that she "never felt so close to God" in the moments leading up to her attack.
At first, she refuses to talk about the attack to the police, but soon, she comes forward, letting herself be processed as a victim and describe her attacker. Her description leads the police to a Russian ship docked at the pier, where they arrest the women's attacker.
In Series Six, Sister Mary Cynthia is sent to the Mother House to convalesce after her attack. However, Trixie stays at the Mother House for a night upon her return to Africa and later tells the midwives that she did not see her there.
Later, Fred and Sister Monica Joan find Sister Mary Cynthia residing at the Linchmere Hospital, an austere mental institution, after they go to assess it for suitability when Fred's cousin with Down Syndrome needs a new home. Sister Monica Joan is severely affected, and frequently finds her way to the hospital to pray at the gates. She later undertakes a food strike/fast in solidarity with her Sister.
Due to the efforts of Sister Julienne and Dr. Patrick Turner, Sister Mary Cynthia is released after receiving electro-convulsive therapy (ECT). However, she continues to suffer very badly from depression, and it is believed her feelings of extreme guilt through not being a "good nun" are contributing to her illness. She is therefore willingly sent to the mental hospital that Patrick himself recuperated at after the war, which seems a much more therapeutic environment. Upon arrival, she asks to be called simply Cynthia by the treatment team.