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Supporting characters in stories have much the same role as supporting actors do. Without them, the story is incomplete. They add richness and depth to a story too, especially if they are involved in the sub-plots in novellas and novels. But their task is to either help or hinder the lead characters in their task. They don’t overshadow the leading character either.
For a villainous character, their supporting character can help increase the difficulties faced by the hero/heroine. That in turn increases the tension in the story and the risks of failure for the hero/heroine. For a more noble character, their supporting characters can help them achieve their objective quicker and limit the risks faced by the lead and increase their chances of success.