Antonyms are words that have the opposite meaning of a certain word. For the word "antithetical," which means directly opposed, some antonyms include compatible, harmonious, and sympathetic. These words suggest agreement or alignment, rather than opposition or contradiction. Other antonyms may include complimentary, complementary, and cohesive, all of which imply a sense of agreement or unity. It is important to remember that antonyms can depend on the context and usage of the word, and may not always directly oppose the meaning of the original word. However, having a strong vocabulary of antonyms can help expand one's ability to understand and use language effectively.