Ask HN: What makes a founder?

The topic of solo-founder vs. co-founder keeps coming up (I'm a solo-founder) and I'm wondering what makes a person a 'co-founder' vs. a 'key hire'. What are the traits, actions, etc. that would separate one from the other? I would expect any key-hire to take ownership and responsibility for their job and be committed (with equity of course), so ignoring compensation differences, what makes a founder?