What do you call a partner who shares ownership and builds the company together?

A partner who shares ownership and builds the company together is termed a co-founder. This designation reflects both a significant role in the inception and development of the business as well as a shared responsibility among the founding members. Co-founders not only contribute capital but also bring skills, ideas, and a vision for the company, which are crucial for its creation and growth. Their collaborative efforts are essential in shaping the business model and driving strategic decisions from the outset.

While a shareholder owns a portion of the company, they do not necessarily participate in its day-to-day operations or decision-making processes. An investor may provide funding but may not have an active role in the company's founding or operation. Similarly, an advisor offers guidance and support based on expertise but does not share in the ownership or operational responsibilities in the same manner as a co-founder. Thus, co-founders are integral to both the strategic direction and operational execution of a startup.