What term is used for an individual afforded the highest trust and confidence while acting on behalf of another?

The term that describes an individual who is afforded the highest trust and confidence while acting on behalf of another is "fiduciary." A fiduciary relationship exists when one party is entrusted to act for the benefit of another, implying a legal and ethical obligation to put the interests of the other party above their own. This includes responsibilities such as acting in good faith, prudently managing the entrusted duty, and maintaining confidentiality.

In contrast, an agent acts on behalf of another party (the principal) but may not always have the same level of fiduciary responsibility, depending on the context of their relationship. Similarly, a sub-agent refers to someone authorized by the agent to assist in carrying out tasks, and their trustworthiness is typically derived from the agent’s authority rather than a direct relationship with the principal. Lastly, while a principal is the entity that grants authority to the agent, they do not represent a party acting on behalf of someone else but rather the one who delegates authority. Thus, 'fiduciary' perfectly encapsulates the highest level of trust and responsibility in such relationships.