Lady Sybil Cora Branson (nee Crawley) was a character on the series, Downton Abbey. She was played for the series first three seasons by Jessica Brown Findlay.
The rebellious, but very sweet youngest daughter
Sybil was the youngest of the three daughters of the Earl and the Countess of Grantham. She had two sisters, Lady Mary Crawley and Lady Edith Pelham. Unlike her more straitlaced sisters, she was quite rebellious, but she was also one of the sweetest people in the world.
Her two grandmothers, who were still living, were her paternal grandmother, Violet Crawley, the Dowager Countess of Grantham, who lived nearby the abbey in the village's Dower house; and her maternal grandmother, Martha Levinson an American who divided her time between New York City and Newport, Rhode Island.
She was very political. Her beliefs were very liberal (kind of borderline socialist), unlike her conservative parents. However, she was very close to her mother. Cora clearly adored Sybil and considered her to be her baby.
Her relations with her sisters were mixed. Sybil and Edith, while friendly towards one another, were kind of distant, although they loved one another. Her closest bond was with her sister, Mary. Mary and Sybil were close, and the former always called her "Darling".
She isn't afraid to get her hands dirty
She knew that her world, as she knew it growing up, was ending, and so had to find her niche in life. One of her closest friends, having been killed in the war, solidified her decision to want to do more. True to her rebellious nature, though, she also felt stifled as the daughter of an Earl and Countess. She wanted to do something she could be proud of.
At the suggestion of her cousin, Isobel Crawley Grey, she decided to try to become a nurse. As a part of that training she was to take, she even learned how to cook with the help of Mrs. Beryl Patmore, the family's cook, and her assistant, Daisy Mason.
Since Mrs. Patmore rarely had any contact with the upstairs family (except for Cora, and that was usually only when she went over the menus with her), she talked to Sybil as she would have had to another kitchen maid, although she would catch herself at it. Daisy, who was more sympathetic to Sybil, helped her along every step of the way.
Her first food creation was a cake they had for dinner. While the butler, Carson, was not impressed, her mother was quite proud of her. Sybil was quite pleased as well.
In the first season, Sybil was very instrumental in helping housemaid, Gwen Dawson, in finding a new job outside of service. She lent her clothes, kept abreast of jobs opening up, getting her to interviews.
In fact, when Gwen got her first job, she, Tom Branson, the family chauffeur, and Sybil were jubilant. After leaving service, Gwen kept in touch with Sybil and Tom through Christmas cards.
She studied to be a nurse in York and her first main posting was at Downton Cottage Hospital. She would also later help out when her house was turned into a convalescent home for war wounded soldiers. During this time, she and her mother shined in their choices of work.
Also during this time, she also became close friends with former footman, Thomas Barrow, who was running Downton at that time. While Thomas did not approve of Tom marrying Sybil, that did not stop him from respecting her and considering her an equal. Sybil was one of the few people (her daughter and her nephew would be two others) that he genuinely cared about.
Meanwhile, Sybil had accepted the proposal of Tom, whom she had fallen in love with, and they married, only attended by her sisters. Robert was infuriated, and would not attend, while Cora, who was saddened, wasn't able to attend, although she dearly wanted to.
Sybil's tragic death
During the third season, Jessica Brown Findlay did not want to return to the show (in the UK, actor's contracts are only for three years), as she wanted to explore new possibilities in her acting career.
As such, she wanted to have her character killed off, so there would be no chance for her to return. The character was killed off due to Sybil having passed away from pre-eclampsia, after she gave birth to her daughter, Sybbie Branson.
Her death devastated everyone in Downton Abbey, but most devastated by her tragic death, were her mother, her sisters, Thomas, and Tom. Cora reaffirmed that Sybil would "always be my baby. My beauty, my baby." Cora, Tom and Mary feel that Sybil's spirit lives on in her daughter, Sybbie. They all see her as the one tangible link that they have to her.
Thomas, in one of his more sincere bouts of grief (which were rare, because of how underhanded and scheming he was), truly mourned her, and his compassion and love that he had for Sybil would extend to her daughter, Sybbie, of whom he was quite protective of (in fact, he had the children's nanny, Nanny West, sacked because of her maltreatment of Sybbie).
Tom had to take over, being both mother and father to their daughter, although Sybil was never forgotten by the people, both above and below stairs, who had been touched by her kindness.
In her final appearance in the show's final season, Gwen, who was visiting the Crawleys with her husband, John Harding for a luncheon, revealed to the family how much she owed her successful life to Lady Sybil. The family felt that it was a perfect testimony to the sweetness and kindness of Sybil and that she was, as Mrs. Hughes said, "the sweetest spirit under this roof".