So firstly, 누가 is a contraction of 누구가, so essentially the only added difference is 누가 has an added "가" to show the subject of a sentence. In terms of usage, 누구 stays as 누구 when it's being used as the object (whenever any verb is doing anything to it). So, if someone "loves" 누구, or "is looking for" 누구, then something is *being done* to 누구 and it stays as 누구 (you would add the particle 를 to mark that it is an object as opposed to 가). If, however, 누구 is the one doing something, then you'd add 가 and it becomes 누가 as you wrote. So if 누구 "loves" someone, or if 누구 "is not here" then that 누구 is doing something and therefore becomes the subject of the verb and needs 가, becoming 누가.
Hope that was helpful.~^^