Apple was co-founded by Steve Wozniak, the engineer who left the company in 1985.
Wozniak designed the Apple I and was the principal designer of the Apple II, two early computers that helped establish Apple as a significant personal-computer company. His engineering work complemented Steve Jobs's focus on product presentation, business strategy, and marketing.
Wozniak remained involved with Apple after a 1981 airplane accident caused serious injuries, but he formally left the company in 1985. He later returned for occasional events and retained an honorary employee role. This answer is distinct from Apple executives such as Tim Cook and John Sculley, neither of whom co-founded the company. Wozniak's technical contribution is especially associated with the Apple II's integrated design and practical usability.