Who is Sibyl from Wasted: A Memoir of Anorexia and Bulimia and what is their importance?

For a short while Marya shares an apartment in Minnesota with Sibyl, a friend from the university newspaper. Sibyl tells Marya her habit of checking out her rear end each day in the apartment's full length mirror is ridiculous. Later at her parents' Christmas party, Sibyl shows concern about how thin Marya has become. Although Sibyl insists that Marya is sick, Marya tells her friend she is fine.