Olive Sinclair was a Dhampir Guardian, and the younger half-sister of Moroi Nina Sinclair. She abandoned her education in her last year of schooling to guard her father who needed protection. This eventually lead to Olive being turned into a Strigoi when protecting her father from a group of them. Her sister Nina found her and then turned her back into a Dhampir state. Olive is seen to have a romantic interest in Guardian Neil Raymond, with whom she had a Dhampir child Declan Raymond. She died shortly after giving birth to her son.
Before the events of The Fiery Heart, Olive had been training to become a Guardian. When her father's Guardian was killed in action, leaving him without a Guardian because the family wasn't royal, Olive left school and took over the task of protecting her father when he went on business trips. In return she agreed to take other means of schooling. On one of her father's trips, they were surrounded by Strigoi. Olive told her father to run away, leaving her to fight them off alone. Unfortunately Olive was overcome and eventually turned into a Strigoi.
Her sister Nina had found her in Dallas, Texas, where with her Dhampir friend James was able to turn Olive back into a Dhampir through Spirit magic. They were soon visited by Guardians, including Rose Hathaway and Dimitri Belikov, who helped Olive recover from her traumatic experience. They were then visited by another Spirit user, Adrian Ivashkov, and Guardian Neil Raymond. Olive gave some of her blood to Adrian because of her blood holding Spirit magic from the transformation from Strigoi to Dhampir. Hhe told everyone that she was willingly to give more of her blood to help the Moroi and Dhampir community from Strigoi. Adrian was impressed by her bravery and told her that she was a brave girl. Olive also seemed to have a romantic attraction to Neil Raymond, who in return saw her as a brave and selfless woman.
Olive was taken to Moroi Court along with her sister for further testing. It is later seen that Neil and Olive had been emailing each other regularly. At Court Olive was a witnesses when Neil had the tattoo and was seen to stare at him adoringly and they talked to each other throughout the trip.
She is eventually named as her sister's Guardian.
Nina is trying to reach out to Olive in dreams, but Olive uses sheer willpower to take control of the dreams from Nina and distracts her with obstacles while she wakes herself up. Nina enlists Adrian's help to find Olive, and the last time they dreamwalk, Olive forces the dream to reflect the Strigoi attack when her and Nina's father was attacked. After Adrian takes back control of the dream from Nina, it's revealed Olive's pregnant.
Sydney, Adrian, Rose and Dimitri take a roadtrip to the Wild Pine Intentional Community to find her, and Adrian informs her about Nina's condition after the dream. The community is attacked by Strigoi as Olive goes into labor. She flees into the woods to give birth to Declan but is attacked by a Strigoi. Before she dies, she tells Adrian that Neil is the father of her baby and that Declan was probably conceived because of the spirit in her from her restoration. Her dying request is to take Declan to Neil and to keep Declan away from Moroi authorities and the Alchemists who would want to experiment on him.