Annemarie is, to put it bluntly, Miranda's replacement for Sal when he stops walking home from school with her. Miranda had never thought much before of the girls at school (11.8), but she decides to pursue a friendship when she learns that Annemarie is at odds with her own best friend, Julia.
While Miranda's relationship with Annemarie might seem at first to be less than sincere, we see Miranda grow into an actual friendship with her over the course of the novel. Annemarie's presence prompts Miranda, for better or worse, to see her life through others' eyes – as when Annemarie and her father visit Miranda's dingy apartment in Chapter 35.
As the two girls grow close, it becomes clear that Annemarie has hopes and feelings just like Miranda. As her character is more fully fleshed out, we (and Miranda) find out that she's a budding artist, and she suffers from epilepsy. She's also fiercely loyal to her ex-friend Julia, which causes a fight between Miranda and Annemarie right before Christmas break (Chapter 37).