In this talk, I examine the distribution of demonstrative ku in Korean. There are a substantial number of studies on the semantics and pragmatics on ku as an anaphoric definite (e.g., Lee 1989, Chang 2009, Anh 2017, Kang 2021, Kim 2023). However, a syntactic analysis of ku as an anaphoric definite is scarce, and the syntactic role of ku remains unexplained. Building on the previous proposals in which ku indicates anaphoric definiteness, I provide a syntactic analysis for the anaphoric definite ku. I propose that the demonstrative ku realizes IndexP (idxP) and it merges in the specifier of DP giving rise to an anaphoric interpretation. Indices are independent syntactic objects and head their own functional projection within the DP, namely idxP (e.g., Schwarz 2009, Hanink 2021, Jenks and Konate 2022). Interesting consequences of the proposed analysis is that it can account for an interaction between number and anaphoric definites in the language. In the anaphoric definite context (ku haksayng*(-tul) ‘those students’), the plural -tul is obligatory in contrast to the indefinite context in which the plural is optional (e.g., haksayng(-tul) ‘students’). I propose that the plural -tul is the realization of grammatical number such as Num in an anaphoric context licensed by ku. Num has binary features [±plural] that value uninterpretable number feature on D [uNum]. As the anaphorice definite ku as idxP merges in the specifier of DP, the obligatory number distinction in the anaphoric context is accounted for. I extend the proposed analysis to other indexed definites such as pronouns and deictic demonstratives in the language, and show that they are also the realization of idxP.