The phrase "be a foil to" means to contrast with or serve as a complement to someone or something. Its antonyms include to blend in, conform, harmonize, or match with. When you want to be a foil to someone, you intentionally take up a different stance or position to complement the other person's actions. However, if you want to go along with the person or if both of you share the same perspective, then you blend in, harmonize, or match with the other person. Using antonyms to "be a foil to" can help someone establish rapport and connect better with others.