Being a foil to means to serve as a contrast or a complement to something or someone else. Antonyms for being a foil to could include blending in, conforming, agreeing, or matching. To blend in means to become one with or to fit in with a particular environment or social setting. Conforming means to abide by or comply with the norms or expectations of a group or society. Agreeing means to share similar views or opinions with someone or something else. Matching means to be identical or similar in nature or appearance. All of these antonyms suggest a lack of contrast or differentiation between two entities, as opposed to being a foil to which implies a clear distinction.