Anna is Gemma's bestie, and based on the limited details we get, the two are pretty tight. We know Anna and Gemma usually spend summer vacations together and that this particular summer is "the first one Anna and I had ever spent apart, ever" (26.9). Furthermore, whenever Gemma imagines what her parents are doing back home in their efforts to locate her, Anna is always there, "standing behind the couch with her hand on Mum's shoulder" (9.7). They're pretty small details, but we can sense from this a pretty close relationship.
Or, can we? While Anna and Gemma have a strong bond, there's evidence that all is not well in their friendship. For one thing, Anna is away for the summer because she's spending her vacation with Ben—her boyfriend whom Gemma has a crush on. Gemma even confesses to fantasizing about him, with his "floppy, sun bleached hair" (26.8), being with her in her bedroom at Ty's.
What's more is that even Ty seems to pick up on the fact that something is not quite right with them. "Anna knew how you felt about him," he tells Gemma. "She didn't like it … she didn't trust you because of it" (37.91). She may not appear much in the story, but Anna is evidence that back home, Gemma has a normal teenage life that she's desperately missing.