I'll add to the growing number of people who already have mentioned this. Approach a potential co-founder from the standpoint of a personal relationship.
Study the history of successful startups. You'll see that the co-founders started as friends. Some are even romantic partners in life. What they share is a history that began before the startup.
There's a huge benefit to this.
Your professional and personal lives will intertwine. So, it's not just about the synergy of how you can work together. You must be confident that you can live together.
We know that Mark Zuckerberg co-founded Facebook with his college roommates. Nathan Blecharczyk co-founded Airbnb with Joe Gebbia after they became roommates. Mutual friends introduced Dennis Crowley to Naveen Selvadurai. Two years later, they co-founded Foursquare.
Startups tend to be all about "right now." But they must begin with a solid foundation. That foundation is a history between co-founders.
You need this. Investors look for it, too.
Consider these two bits of advice from Peter Thiel's book, "Zero to One."
Peter Thiel and other successful VCs want to see a considerable history between co-founders. They want to see it for the same reason you need to have it. The relationship between co-founders is predictive of a startup's viability.
Does this mean you're out of luck if you don't have a longstanding relationship with a co-founder? Far from it. That's what startups are all about. Defying convention. The co-founding team is only a part of what will determine your success.
Just know that it's one of the things you have to get right. It's more than a test project of a couple months. It's more than weekly check-ins. It's more than establishing precautions to protect yourself and your company.
All these things are for employees. Not co-founders.
Meet up with this potential founding team member, Andrea. Spend as much time as you can with them. If you don't have history, create it.