"누가" is short for "누구+가(subject particle)" meaning it's a subject, but the pronoun "누구" can be a subject like "누가", object, topic or predicate in a sentence. e.g. 누가(=누구가) ☞ as a subject 누구 + 는 ☞ as a topic 누구 + 를 ☞ as an object 누구 + 예요 ☞ as a predicate 누구 + 의 ☞ as a possessive 누구 + 에게 ☞ to who(m)