The word "counteragent" refers to a substance or person that neutralizes the effects of another substance or person. Antonyms for this word would include terms that describe something that enhances, promotes or supports something else instead of neutralizing it. These could be words like "catalyst," which refers to a substance that speeds up a chemical reaction, or "ally," which describes someone who supports or aids another person or group. Other antonyms for "counteragent" could include terms like "facilitator," "promoter," "booster," and "advocate," which all suggest a positive influence or impact on a particular situation or outcome.