If someone asks, "Who is this?" it typically means they don't recognize or know who you are, and they are seeking clarification. In this situation, you can respond with a brief introduction or explanation of who you are. For example:
Option 1: "I apologize for not introducing myself earlier. I'm [Your Name], [briefly mention your role or context]. How can I assist you?"
Option 2: "I believe we haven't had the chance to meet yet. I'm [Your Name]. Nice to meet you! How can I help you?"
Remember to keep the response polite and friendly, as the other person might be genuinely curious or trying to figure out your identity in a friendly manner.