With the large influx of Russian-speaking migrant workers and their children to South Korea, there is a growing need to explore the language and literacy practices of emerging multilingual learners. Specifically, as the Korean language is predominantly used in South Korean contexts, including school, Russian-speaking children face enormous linguistic and academic challenges and demonstrate different language practices in and out of school contexts. Drawing upon the conceptual frameworks of translingualism and agency, this three-year longitudinal case study attempts to investigate how and for what purposes an emergent multilingual youth executes his agency to engage in translingual practices within and across different contexts (in and out of school, and in an after-school program called Translingual Writing Program) and how his participation in translingual practices influences how he exercises his agency as a language learner. This study employs multiple sources of data to build a multifaceted perspective on an emerging multilingual youth's translingual practices and agency across multiple contexts over the course of three years. The findings suggest that the youth's engagement in translingual practices became more agentive and active under circumstances in which his multilingual beliefs and competence were valued, and where a relatively large number of resources were available.