In the beginning Yui acted in a striked, somewhat bossy way to practicaly every character, most often Rito.
Over time she became more friendly towards others after observing Rito and learning of his true kind nature. She has since been developing fellings for him little by little over the serise.
By the later portion of the first serise she had unconsciously fallen in love with Rito, and by Darkness she had admitted it to herself. Her love for Rito has been a major influence in her actions. Often tryinng to become more friendly with him and learning more about him by reading his fathers manga and talking to him about it. She has come to understand that although he is interested in sexual actions, all his perverted incidents have been by complete accident, not on purpose, and has become less angry with him when this happens.