This chapter explores how linguistic, cultural, and ethnic identities are displayed, intertwined, and negotiated by YouTube creators in their reaction videos on BTS content. The reaction video, “a meta-video that documents in real-time individuals watching another video usually for the first time,” has emerged as a popular genre of user-generated content and a venue for online participation on YouTube for K-pop. Employing multimodal analysis, the chapter focuses on one YouTube channel run by a Korean American reactor and the comments and interactions made by the reactors and viewers on various aspects of BTS content. The analysis focuses on how reactors construct and perform their fan identity and position themselves vis-à-vis BTS in terms of language, culture, and ethnicity, including the hybrid language use of Korean and English and cultural aspects observed in BTS content. It is argued that reaction videos function as a new third space, in which the reactors’ different identities are (re)constructed and mediated as developing YouTubers and Korean American fans, and linguistic norms and cultural hierarchies are reordered and renegotiated.