Throughout the book, Kaysen portrays the nurses who work at McLean as foils opposite her and her fellow patients. Here is one example from Chapter 21:
The student nurses were about nineteen or twenty: our age. They had clean, eager faces and clean, ironed uniforms. Their innocence and incompetence aroused our pity, unlike the incompetence of aides, which aroused our scorn. This was partly because student nurses stayed only a few weeks, whereas aides were incompetent for years at a stretch. Mainly, though, it was because when we looked at the student nurses, we saw alternate versions of ourselves. They were living out lives we might have been living if we hadn't been occupied with being mental patients. They shared apartments and had boyfriends and talked about clothes. We wanted to protect them so that they could go on living these lives. They were our proxies.
At a different point in Chapter 21, Kaysen points to the foil very clearly: "Mainly, though, it was because when we looked at the student nurses, we saw alternate versions of ourselves." The nurses stand in for everything the patients have been missing out on because of their mental illness or because they've been deemed "crazy." While Kaysen writes about herself and the other patients in a way that highlights the chaos and disorder of their lives, she remarks on the "clean, eager faces and clean, ironed uniforms" sported by the nurses. They're also similar in age to the patients and are experiencing life and career milestones that the patients must delay while they're at McLean. The nurses represent what life might look like for the patients if they didn't suffer from mental illness—as Kaysen writes, they act as "proxies" that underscore the difficulty of the patients' lives.