To my mind, a co-founder is someone who directly contributes to not only developing the solution to the problem you're facing, but choosing the problem as well.

To my mind, a co-founder is someone who directly contributes to not only developing the solution to the problem you're facing, but choosing the problem as well. You've already decided on the problem domain (investments), how you're going to solve it (SaaS), and have an understanding of the potential growth models you'd look for (bringing someone in to ramp up sales). Of course, if you're open to someone only working one day a week on it to come in and challenge all of that and potentially pivot into something else, then they might be a co-founder. But from what you've described, they're a very senior hire (and might not be right, with sales you want to hire for passion, rather than academic smarts), rather than your co-dependent. I was employee number 3, but I certainly wasn't a co-founder.