This paper examines the complement as a sentence element in Korean grammar. In school grammar, only `되다(doeda)` and `아니다 (anida)` NP2 are designated as complements. However, the NP2 of psychological adjective constructions can also be classified as complements in addition to `되다(doeda)` and `아니다(anida)` NP2. This study defines `가(ga)` complements as all arguments required by the predicate in addition to subjects and objects, and describes their characteristics. The so-called `mandatory adverbial` cannot be considered a complement because it shows characteristics different from `가(ga)` complements. The characteristics of `가(ga)` complements can be summarized as follows: 1) they are mandatory; 2) they comprise theta-role themes; 3) they cannot form the head noun of relative clauses; and 4) omission of the attaching particle ` 가(ga)` is permitted. These four characteristics of `가(ga)` complements can be used to identify constructions that require a ` 가(ga)` complement. Thus, by clarifying that within the Korean grammar system a `complement` may have sentence element characteristics that differentiate them from subjects, the results of the current study suggest that nominative case particle `가(ga)` should be considered a separate homonym of complement case particle `가(ga)`.