Well, an operating role is just a term that separates someone from being just a highly paid advisor. I’m assuming here all parties are reasonable about what they give equity for and we’re talking about a start up that can really go big.

In this case, if someone is getting even 1%+ equity (even on a on a vesting schedule), they’re likely contributing a lot more than just mockups. So if they’re doing something, it’s early stage for little or no pay, I think the point where someone is getting close to 10% is a signal that they’re contributing enough to make them a co-founder.

I don’t think replaceability matters in defining a co-founder. Talent is usually replaceable. Being a co-founder is more about being able to assume significant risk in the face of uncertainty to build a business. Just because someone contributes a lot to lifting a business off the ground doesn’t mean they’ll do the same later, or contribute to it's "longevity." Many times someone that works well in a 1-3 person start up might not fit as well as a manager or when the company has 500+ employees. However that still doesn’t take away from them being a co-founder.