Based on the meaning of someone who acts on behalf of another:
* Principal: The person or entity for whom the agent acts.
* Client: The person or entity who is served by the agent.
* Boss: Someone who gives orders and makes decisions, rather than following them.
* Independent: Someone who acts on their own behalf, not for anyone else.
Based on the meaning of someone who causes something to happen:
* Patient: Someone who passively accepts events, rather than influencing them.
* Observer: Someone who watches and records events, rather than causing them.
* Inert: Something that is inactive and has no effect.
Based on the meaning of a chemical or physical substance that causes a change:
* Catalyst: A substance that speeds up a chemical reaction without being consumed itself.
* Inhibitor: A substance that slows down or prevents a chemical reaction.
To find the best antonym for "agent", you need to consider the context in which it is used.