|Portrayed by Emilee Wallace|
|Parents:||Hope Martin (mother)|
|Siblings:||Maura Isles (half-sister)|
|First appearance:||"Money Maker"|
|“||"Cailin" means "uncertain." That's the name that my mother picked for me. "Maura" means "great." That's the name she picked for you.||”|
–Cailin, "Money Maker"
As a child, Hope seemed to have told Cailin about her stillborn child often. Although since Hope was a medical activist, she was often busy and away. At one point, Cailin contracted a bacterial infection while Hope was doing relief work in Africa. Unable to find a kidney to match hers, Hope moves to Boston with her. Since finding out, Hope has been trying to bond with her to make up for all the years she wasn't there. Caitlin, however, has grown more distant and has already accepted the fact that she is dying.
When they move to Boston, she openly shows her contempt about Hope's decision to move to Boston.
During an awkward dinner at Maura's with Hope, Jane and Angela, Cailin goes snooping around in Maura's bathroom and finds Paddy's drawing of Hope. She then starts looking up Maura and finds out that her father is Paddy Doyle; she then connects Maura to Hope's dead daughter.
When Maura later finds out that Cailin needs a kidney, she decides to donate one of her own anonymously.
Later, Cailin comes back to Maura's house and confronts her, telling her that she knows that she is her mother's child, too. She tells Maura that she has lived in her dead shadow her whole life and she never seemed to have been enough to fill the hole in her mother's life. Cailin also figures out that Maura was the anonymous donor of the kidney and she insists on not accepting it anymore, even if it meant that she will die, saying that she does not want any part of Maura in her.
During the argument, Cailin also points out that she should have just told them the truth. So later, when Hope suddenly arrives, Maura tells Hope the truth out of nowhere, pressured by Cailin's breakdown. Cailin seemed shocked that Maura did what she did, even more when her mother retreated from the scene with her, saying that Cailin is her daughter, and even seemed reluctant to leave Maura in tears when Hope wanted to leave, pulling Cailin along with her.
It is revealed that, despite Cailin's objections, Maura still secretly donated her kidney to Cailin. Upset, thinking that she and Hope were ungrateful, Maura tails her, hoping to find out how Cailin is treating her new kidney, and when Cailin sees her, she thanks her, admitting that she knows. She then gives Maura a letter and the scrimshaw necklace, the Doyle family heirloom that Hope gave her on her 18th birthday (which Hope had apparently received from Paddy on her 18th birthday), saying it belongs to her. Cailin then offers to keep in contact with her, asking if she can "text her" sometime, and Maura happily agrees.