(1) Background: Adolescents admitted as acute inpatients belong to a particularly psychosocially vulnerable population. This study aimed to examine the clinical characteristics of an affected population in Germany using a theory-based approach. (2) Methods: We assessed the mental health problems, levels of personality functioning, and the severity of social withdrawal and loneliness in n = 62 adolescents admitted to an acute psychiatric inpatient unit. Cases were investigated cross-sectionally utilizing standardized psychometric questionnaires from the perspective of the patients and clinical experts. (3) Results: Mental health, level of impaired personality functioning, social withdrawal, and loneliness were all positively associated with the need for acute admission. Further analyses revealed that the level of personality functioning fully mediated the positive association between social withdrawal and mental health problems. In contrast, level of personality functioning only partially mediated the positive association between loneliness and mental health problems. (4) Conclusions: Our results suggest that more impairment in personality functioning might lead to poorer mental health when adolescents socially withdraw in the aftermath of the COVID-19 pandemic. Loneliness, social withdrawal, and the level of personality functioning may help identifying essential characteristics of adolescents admitted to acute psychiatric inpatient units and guide the development of specific interventions.
