The present study used quantitative and qualitative methods to explore how lonely young people are seen from others' perspectives, in terms of their personality, behavior, and life circumstances. Data were drawn from the Environmental Risk Longitudinal Twin Study, a cohort of 2,232 individuals born in the U.K. in the mid-1990s. When participants were aged 18, they provided self-reports of loneliness, and informant ratings of loneliness were provided by interviewers, as well as participants' parents and siblings. Interviewers further provided Big Five personality ratings and detailed written notes in which they documented their perceptions of the participants and their reflections on the content of the interview. In the quantitative section of the article, regression analyses were used to examine the perceptibility of loneliness and how participants' loneliness related to their perceived personality traits. The informant ratings of participants' loneliness showed good agreement with self-reports. Furthermore, loneliness was associated with lower perceived conscientiousness, agreeableness, and extroversion and higher perceived neuroticism. Within-twin pair analyses indicated that these associations were partly explained by common underlying genetic influences. In the qualitative section of the study, the loneliest 5% of study participants (N = 108) were selected, and thematic analysis was applied to the study interviewers' notes about those participants. Three themes were identified and named: "uncomfortable in own skin," "clustering of risk," and "difficulties accessing social resources." These results add depth to the current conceptualization of loneliness and emphasize the complexity and intersectional nature of the circumstances severely lonely young adults live in.