がSubject and identifying marker
が marks the person or thing that acts, exists, or has the state being described. It is especially common when that subject is new, unknown, or being identified.
What the marker points out
The noun before が is the person or thing directly connected to the action, event, existence, or description. This is natural when you notice something happening or introduce someone or something that has not already been established.
Actions, existence, and states
Asking and identifying who
The question asks for the unknown subject, and the answer identifies that subject.
Subject or established topic?
が identifies or introduces the person or thing directly connected to what follows. The topic marker は instead establishes what the sentence is about; it is written は but pronounced わ. In a question asking who or which one did something, keep the question word and the direct identifying answer marked with が; changing it to the topic marker changes the job of the phrase.