the only lead
From the beginning, it's very obvious that Riku does not trust Maleficent at all. He knows she has some ulterior motive for helping him out, but because he can see no other way to rescue Kairi, he cooperates with her. Using her manipulative ways, she tempts him with dark powers, and he accepts them because he thinks they will help him bring Kairi back. His only purpose is to save Kairi; he cares for nothing else. He is using Maleficent just as she is using him. In the end, after Maleficent is defeated by Sora & Co., Riku-Ansem stabs her with his Keyblade, turning her into a dragon. She is defeated once again by Sora & Co., and that is the end of her... or so we think.
She returns in Kingdom Hearts: Chain of Memories as part of both Sora's and Riku's memories. She tries her hand at tempting Riku again, but he defeats her. Then in Kingdom Hearts II, she unexpectedly returns with a new accomplice at her side Pete. However, she no longer has any interest in Riku, so they don't interact much at all.