[Not my picture]
Listen to A Thousand Years by Christina Perri:
Celeste was never attached to her biological parents. She was not properly loved by her adopted parents, and so she never had a sense of belonging or purpose. That is, not until the rebellion. She found allies in the cause, and although most were never friends, it gave her the motivation and direction she needed in her life. When she met Fiona, she was instantly aware of their connection, and wanted a sisterly relationship, despite her being jealous of Fiona's spoiled childhood.
Rico is introduced to the story some time after Celeste had to have met him. They likely met through the rebellion, and as the rebels grew stronger so did their relationship. They became the side story couple, playful and "younger" than the main characters.
The way Celeste communicates with those around her reflects that of her half-sister, Fiona. This is most probably because Celeste has never had any role models or family members before, so she takes on Fiona's style, trying to regain the years of copycatting and personality building that she didn't get as a child. She's trying to fill that void. If you pay attention you can see it: The way both Celeste and Fiona are serious but playful with the guys they like, the way they both speak and lead with power. These are the signs of under-fulfillment as a child.
After Abraham dies, and Fiona becomes irrational, Celeste doesn't know how to help Fiona, as she had never had someone to take care of besides herself.
When Fiona sends her away, she is grateful to have Rico as her side, but has mixed feelings about leaving Fiona in her declining mental state.