Patients with eating disorders (EDs) often spontaneously talk about their disorder in terms of a symbolic other (a demon, a voice, a guardian). Further, externalizing exercises where patients are encouraged to separate their true self from their ED self are common in some treatment approaches. Yet, no previous quantitative study has investigated this phenomenon. We examined the patient-ED relationship (using the interpersonal structural analysis of social behavior methodology) and its implications for ED symptoms, illness duration and self-image. Participants were 16–25 year old female patients (N = 150) diagnosed with anorexia nervosa (N = 55), bulimia nervosa (N = 33) or eating disorder not otherwise specified (N = 62). Results suggested that patients had comprehensible and organized relationships with their EDs. EDs were primarily experienced as acting critical and controlling towards patients. Higher ED control was associated with more ED symptoms and longer illness duration, especially when coupled with patient submission. Patients reacting more negatively towards their EDs than their EDs were acting towards them had lower symptom levels and more positive self-images. Externalizing one's ED, relating to it like a symbolic other, seemed to make sense to patients and depending on its quality seemed to influence ED symptoms, illness duration and self-image. We put forward both clinical and theoretical implications based on the assumption that the patient-ED relationship may function in similar ways as real-life interpersonal relationships do.