가

Marks the subject, after a vowel

Marks the word that is the subject of the sentence. This is the shape used after a vowel; 이 is the identical marker after a consonant. Set against 은/는, which frames what the sentence is about, this one singles out the specific thing that does the action or fits the description — it is the marker in "this is the one that's south" and in reports of something newly noticed. Speakers also drop it entirely in fast casual speech when the meaning is obvious.