Agent

An agent is any entity, conscious or not that is the sole responsibility of some controlling principle.

Agents have a legal definition, and a more metaphysical definition.

According to the Cornell Law School Legal Information Institute

An agent is a person authorized to act on behalf of another person. The party an agent is authorized to act for is known as the principal. A principal-agent relationship can either be intentionally created or created by implication through one’s actions.

In the more metaphysical sense, an agent does not need to be a "person" as such, but simply any arrangement of matter, or really any imaginary entity that acts on behalf of a controlling entity. If you are playing the board game Monopoly your agent is your game piece. The piece is moved around the board, according to your will, and its actions (within the world of the Monopoly) are done on your behalf.