Sinead Starling is an Ekaterina and is one of the Starling triplets. Along with her brothers, Ned and Ted, they formed Team Four for Grace's competition version of the 39 Clues hunt. She is friends with Amy and Dan, and it was revealed in Mission 11 that she and her siblings became Madrigals. During the first five books of the second series, she is living in Amy and Dan's guest house. She becomes very good friends with Amy in the second series. She also turns out to be Vesper Three, the mole. She had a passionate hatred for the Cahills because of what the Clue hunt did to her brothers and plans to heal them with the serum. However, she turned her back on the Vespers when she discovered that they were going to kill Ted and thus defected back to the Cahills.