The most important question to evaluate in a potential hire is “are they teachable?”.
By teachable I do not mean “we do everything perfectly here, so they must be able to learn from us.” In fact, that attitude would represent the opposite of teachable.
Beyond a desire to learn, being teachable means being humble enough to consider that you might be wrong. And being open to changing your opinion when that’s the case. That’s a quality critical to both the established organization and the potential hire, assuming both sides want to grow. (It’s also important because software developers are usually wrong before lunch.)
The second most important question to evaluate is “are they stubborn enough?”.
Those two qualities appear to be in conflict. But you cannot solve difficult problems without being exactly the right kind of stubborn. (Sure, the words “diligent” or “persistent” could be used here, but “stubborn” captures the required vigor.)
Be teachable, but don’t forget to also be stubborn.