Which adjective signifies a complementary and supportive relationship?

The adjective that signifies a complementary and supportive relationship is "ancillary." This term describes something that provides necessary support to the primary activities or functions. In various contexts, ancillary elements enhance or contribute positively, making them supportive in nature.

"Complementary," while appearing similar, generally refers to elements that complete or go well together in a way that enhances each other, rather than focusing solely on the supportive aspect.

"Antagonistic" suggests a relationship characterized by opposition or conflict, which is the opposite of a supportive nature.

"Indispensable," refers to something that is absolutely necessary or essential, but doesn't inherently indicate a relationship of support or complementarity, as it doesn't imply the nuanced interaction that "ancillary" does.

Thus, "ancillary" is the term that best captures the essence of a supportive and complementary relationship.