Cecilia is the oldest daughter of the family, and she is going to college at the time of the opening scenes of the story. She is back home at for the summer break, and feeling a lot of discontent and restlessness, but not sure why. She has felt uncomfortable and ill at ease at home, and so is snippy and irritable, especially with Robbie. She isn't quite sure why she feels this way towards Robbie, but it becomes clear to her after the fountain incident and the misguided note that he sends to her.
As the story unfolds, it becomes apparent that she is a very passionate person; she vigorously defends Robbie and disowns her entire family after they refuse to support him in the trials. She defends her love for Robbie, showing even more passion on that front. She is bitter and resentful to her family, and fiercely independent. After she rejects her family, she moves out, refuses money from them, and makes her own way in the world through her own hard work.
I hope that those thoughts help you on the character of Cecilia. For themes, try submitting a separate question, as the format of this website allows for one a day. Good luck!
At the beginning of the novel 'Atonement' Cecilia is rather sketchily drawn in terms of personality - this may or may not be deliberate on the part of the writer Ian McKewan. He may be trying to paint her as rather feckless and flighty - a privileged but empty-headed girl who aims to while away her summer vacation at her parents expense at their house in the country. On the other hand, it could be the author failing to develop the character properly - there seems to be a huge chasm for Cecilia to cross into her new territory of falling in love with the man of her dreams and having the commitment to stick by him later in the book. She is obviously a flirty and passionate character who eventually owns up to her attraction to Robbie.
what is the theme of Atonement?