South Korea has long emphasized a strong monocultural identity, and even as immigration has increased and schools have become more culturally diverse, education policies have continued to prioritize assimilating minority cultures into the dominant mainstream (Yang, 2024; Yum & Cha, 2022). This assimilationist orientation has rarely been interrogated, and limited professional or public dialogue about its limitations has constrained efforts to address the complex realities and inequities experienced by culturally diverse students. Our previous study examined four Korean music teachers working with high proportions of immigrant-background students and identified three interrelated themes: (1) fostering inclusive music classrooms through sustained emotional support, diverse musical repertoires, and cooperative ensemble activities; (2) navigating the dilemma between representing students’ cultural identities through music and respecting their wish not to stand out because of their background; and (3) exercising individual teacher agency to overcome systemic gaps by developing new instructional materials, engaging in self-directed learning, and initiating independent music programs, such as culturally diverse school ensembles (Jung, Lee, & Kang, 2025). The study concluded that, although individual teachers play a pivotal role, their efforts are unsustainable without broader systemic support. Building on this foundation, the current study represents Phase 2 of a two-stage research design. Whereas Phase 1 used a multiple case study to identify cross-case patterns, Phase 2 presents an in-depth narrative case study of a single teacher whose practice demonstrates a particularly high level of culturally responsive pedagogy and teacher agency in music education. Through repeated interviews, extended classroom observations, and participation in school music events that he leads, this study offers a rich, situated account of his experiences and professional decision-making. Guided by Culturally Responsive Pedagogy and Teacher Agency theory, the analysis aims to identify concrete strategies, curricular approaches, and contextual conditions that enable culturally responsive music teaching to take root (Biesta, Priestley, & Robinson, 2015; Gay, 2018; Ladson-Billings, 1995; McKoy & Lind, 2022). Findings highlight enabling and constraining conditions at classroom, school, and policy levels. By moving from a broad comparative analysis to an in-depth narrative exploration, this research examines the conditions that currently compel individual teachers to compensate for systemic support gaps affecting students from diverse cultures in South Korea, a pattern that is neither sustainable nor ideal. Practical implications will be discussed for establishing a coherent, context-sensitive structure, including aligned policies and sustained professional development, to support both teachers and students from diverse cultural backgrounds.