Buyer personas are “semi-fictional representations of your ideal customer based on real data along with some select educated speculation about customer demographics, behavior patterns, motivations, and goals.” Basically, a buyer persona is a sketch of your average consumer based on research you have done. Research can be collected through interviews, forms that have been filled out by customers on your website, surveys, and any other number of ways you can find information about your clientele.
Your marketing agency's job is to deliver leads to your sales team. That's what marketing is. Marketing is doing what it takes to deliver leads. This is different from sales. The job of a sales person is to close deals. The job of a marketer is to start them.