Counter-agent refers to someone or something that is against the actions of an agent. When it comes to antonyms for the word counter-agent, we can think of several options such as ally, assistant, helper, supporter, champion, advocate, and backer. An ally is someone who supports or works with an agent towards a common goal, while an assistant or helper is someone who provides aid or assistance to an agent. A supporter, champion, or backer is someone who supports or promotes an agent's actions or beliefs. On the other hand, an advocate is someone who speaks or pleads for an agent's cause or interests, rather than against it.