When it comes to synonyms for "be complementary," there are several options to consider. For example, you could use "supplementary" to indicate something that adds to or enhances another element. "Compatible" may be appropriate when describing two things that work well together without conflict. "Harmonious" suggests a similar notion of things working in tandem, but with a particular emphasis on music or sound. Finally, "synergistic" can be used to describe two elements that multiply or amplify each other's effects. By using any of these synonyms for "be complementary," you can add variety to your writing and clarify your meaning more effectively.