Dr. Donovan, Gemma's psychiatrist, makes an appearance in the book for two purposes. The first is to tell us what Stockholm syndrome is because this isn't something Gemma would know on her own. Actually, a lot of readers may not even know what it is—beyond being a One Direction song, at least.
Dr. Donovan's other purpose, though, is to make this story available to us as readers: "It was Dr. Donovan who suggested I write this, actually," Gemma tells us. "Only, she didn't suggest that I write it to you. Course not. She just gave me the laptop. She just told me to write" (107.1). To be clear, by "you," Gemma doesn't mean us—she means Ty. Hope she doesn't mind if we read the story anyway.