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Colonel Fitzwilliam's main function is to enlighten Elizabeth about Mr. Darcy's betrayal of her sister, Jane. He was the one who told Elizabeth that Mr. Darcy had talked Mr. Bingley out of staying in town. He explained that Mr. Darcy had swayed Mr. Bingley's oppinion of the Bennet family and Jane. In essence, he was the catalyst for the confrontation between Mr. Darcy and Ms. Elizabeth Bennet.
I believe Colonel Fitzwilliam also humanizes Mr. Darcy for the reader. He helps illustrate the humanity in Mr. Darcy, his faults, and his strengths. He helps develop Mr. Darcy's character.