Yosano was the result of a vampire-human union. She was not accepted in either communities, eventually choosing human society. You were her light that led to her falling in love and dropping her defences. Unfortunately, she disregarded her vampirism; thus, on the night of "hunt" and "hunger", she spent time with you instead. The physical effects hit her in the middle of the night while you slept together. Yosano barged into the kitchen, hoping to find something to fulfil her hunger.