ABSTRACT This study explores how a multilingual Hong Kong student exercised agency to maintain a stable Dominant Language Constellation (DLC) amidst significant shifts in sociolinguistic contexts. Employing an intrinsic case study approach, the study collected data through interviews and written documents, which were then analysed using thematic analysis. The findings revealed that the multilingual student strategically managed his language use by compartmentalising Cantonese, Putonghua, and English into distinct domains, a practice he maintained consistently across different environments, including Hong Kong, Beijing, and Montreal. Key sources of his agency were identified, such as his self-conception as an introverted individual, parental influence, and the demands of his educational contexts. His strategic non-participation in social interactions helped him avoid linguistic discomfort and maintain psychological stability. This study contributes to the field of multilingualism by illustrating how agency can be manifested through deliberate non-participation and strategic language use, offering new insights into the dynamic interplay between individual agency and sociolinguistic environments.