This study investigates the dynamic identities of an Asian university student engaged in English as a lingua franca (ELF) interactions from a membership categorization analysis (MCA) approach (Sacks, 1972a, 1989). Studies adopting MCA have demonstrated that identity and intercultural membership are co-constructed in ongoing interactions (e.g., Nishizaka, 1999; E. Zimmerman, 2007). Nevertheless, MCA studies have yet to document the multicultural identity of an individual and the ways in which members co-construct their multifaceted identities in naturally occurring non-institutional ELF interactions. The study analyzes interactions between two participants from different Asian countries, Japan and Korea. Approximately three hours of video recorded conversations were collected across four months. In and through the interaction, one of the participants was found utilizing multiple cultural identities to accomplish interactive goals. In addition, “language-form related category-bound activity” was used in constructing these identities. This study challenges the use of predetermined social categories and suggests an organic and interactional approach to identity construction.