You can easily keep private notes about any agent/query in your query list.
First, make sure you've added the agent to your query list. Once that is done, you'll see a little post-it note icon near the agent's name. It will appear on the Agent Search Page, your Query List Page, as well as the agent's profile page.
Clicking that icon will allow you to create a private note that will be saved to that agent's query.
Anytime you want to view the note, just click the icon again.
See the screen shots below for step-by-step instructions.