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Dohoon's childhood friend and relative (as Dohoon knows it). They lived together since she was 5 years old and were always together. She grew close to him and wanted him all to herself which led to her making him an outcast at school. Then, in a series of bullying cases, Dohoon declared his independence and at the same time he made his wish to transfer, but they met again at the high school he transferred to. 

Actually Yoon Siyeon was a Drinker. So far she had been watching the Dohoon by Salem's command. However, since she was not a Pure Blood Drinker, she could not take out the "pattern" and could not even regenerate. (This is the same with her brother Ciel.