Darcy is a dynamic and round character, which means that he (as a person) changes throughout the story, and so does his storyline as a result of his interaction with the main character, Elizabeth.
He is important in different ways: First, he comes across as the very sullen, aristocratic snob who won't even speak 2 words unless forced to. He sticks to his circle of friends, barely interacts with the country folk, and claims that nothing would induce him to dance. He even claims Elizabeth is too plain to even look at, and all throughout he is hated by everyone.
Meanwhile, this attitude is what made Elizabeth (and the reader) start becoming partial to Wickham not knowing that he is in fact the evil and most corrupt character in the story whose charm makes everyone believe his story about how Darcy destituted him.
Darcy's silence is his most powerful characteristic. His quietness hides facts about him: We realize that he had tried to help Wickham, that he tried to save Bingley from marrying Jane at first out of his own worry for his friend (not because he disliked Jane), that he saved Lydia and Wickham's reputation marrying them off, and that he is, in fact, an honest and good person.
He basically affected everyone in the story with his personality, and with his actions. He certainly was catalyst agent in the lives of many, and he was, himself, a changing individual. He is quite a complete, full character with his dynamics and his roundness.