I've built over a dozen prototypes, spent my own time, money, and energy on them for at least a month in some cases, only to have the non-tech co-founder get cold feet once it's time to market it.
So first off, I want someone who won't do that. Experienced people who have built successful businesses have done that. Normally reliable long term friends and colleagues have done that. People who have more to lose from failure... they're the most likely to back out.
What I like to see is someone dedicated, who really believes in what they're doing. Those who talk only about market and money often back out as soon as an opportunity with a more money comes up... and if they're qualified, they have a non-stop stream of opportunities.
I'd also like to see a somewhat validated business model. Be creative in validating. One of my favourite examples is a friend who wanted to start a car rental business, but before buying the car, he put up ads on the car to check that the market existed.
When someone comes up to me with an idea, I ask them why they haven't built it in a spreadsheet, or WordPress first. One guy did a Airbnb for musician gigs. He had a static site with a profile for each musician with manual payment and people were paying for it.
This doesn't apply to all products, and usually not SaaS. But I want to be able to trust the co-founder to reduce risk, sell, and understand the user/product so I can focus on building stuff. There will also be a lot of pivots. Make sure that the pivots are at least thought out and not too late.