I think, the most important attribute for a startup founder is resilience. Building a startup involves constant challenges, setbacks, and uncertainties. Resilience allows founders to stay focused, adapt to changes, and keep pushing forward despite obstacles. It fuels perseverance during tough times, helps maintain a positive outlook, and drives the determination needed to pivot when necessary.
I agree with @alianjamparuthi, resilience is fundamental! But I would add also the curiosity for learning skills in different domains. Founders have to deal with product development, sales, marketing, customer support and much more.