Designer. Specifically a UX "designer" as you say, who doesn't just draw screens, and won't draw any for weeks or months. They will not just use their knowledge but will go out and talk to users, observe how they solve the same problem today, and help create the scope of work, and design the basic functionality.

If you hire a developer first, they will build stuff that may or may not be of any value. If the company survives the first few rounds of building the wrong thing, you are still spending a hell of a lot in rebuilding, re-marketing, etc. etc.