Appropriate research method to understand the target audience

I need to make a mobile app where the client described the problem, why they need the app, the idea about it, but nothing more. I already understand what kind of features it should contain to solve the problems that the end user experiences without the app (from my experience), but keeping in mind the point, that we need to understand the target user, what would be the best method for this if I can't get to the users? How can I gather this information, so I can validate my assumptions?