The Uchiha Massacre
The day the great tragedy struck. Itachi, such a nice young man, was now classified a criminal because of his crime of murdering his whole clan, leaving only Sasuke alive. As the story goes, Sasuke felt alone. Needing to relive that trauma thousands of times inside his head, and even more after being put out of genjutsu his brother put on him. He wanted to be left alone, but that wasn't possible with Yui always being close to him.
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Sasuke distanced himself. Yui saw how no matter what she did, he would always react coldly. He never wanted to train together, it "distracted" him from his goal. He thought there was no one who understood his pain, but there she was. Standing right beside him the whole time, Yui knew what it felt like to loose everyone. She also knew that if he didn't want her near him, the last thing she should do is force him to accept her near him. She didn't want him to hate her, no... he already had enough hatred inside for his brother. So, Yui did as she was told with no words. She didn't force him to hangout, nor did she make him eat lunch with her. She just stayed. She just was by his side, quietly accompanying him. She sat beside him, saying a few words such as "Good morning" or "Have a nice day". Sasuke knew that even after months of not responding to her, she was still his friend. Yui gave him the feeling she needed, that she will never leave his side, no matter how unbearable he gets.