What is the role of a person who takes assets already in place and keeps the business running?

The role described in the question pertains specifically to a manager. A manager is responsible for overseeing the operations of a business, ensuring that existing resources, including personnel and assets, are utilized effectively to maintain and improve the company's performance. Managers implement strategies, coordinate tasks, and provide leadership to ensure smooth operations.

In contrast, an investor typically provides financial resources to a business without necessarily managing its day-to-day operations. An entrepreneur is involved in starting and building a new business, often taking on the risk of creating something from the ground up rather than managing pre-existing assets. A partner is someone who collaborates in a business, sharing responsibilities and profits, but may not exclusively focus on running the business. Thus, within the context of maintaining ongoing business operations with existing assets, the role of a manager is clearly the most fitting.