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Sayuki Tokihara came from Abusive family
Her family always trying to control her. There is no room for Sayuki to express her own opinions and needs. Love is conditional on being good. Taken to extremes, her parents may abuse, chastise and humiliate her, threatening to abandon or punish if she does not toe the line. So, Sayuki hold onto her hurts, wishing to get back at her parents but lacking the power to do so. Any attempts at revenge are done furtively or passive aggressively. The intrusive or critical behaviour of her parents can become the internal voice of a bullying inner critic. Mao Nanjou was bullied
Because of her interest in yaoi, she was bullied and ostracized by her peers. This made her develop a harsh nature. The reason why she used Keiki and Soma as models for her manga, was because she actually wanted to be friends with both of them, but her experience as socially outcast in her school made her not know how. Yuika Koga's Freudian Excuse
Her personality is a result of having to live in an abusive household, even having to avoid death itself at certain points. Plus, she also had Friendless Background. It affected her mental state so that she got used to the abuse she was going through, turning her into a sadistic person. Mizuha Kiryu's backstory
As a child, she did not get enough attention from her parents, and sometimes she was neglected. That is, until she met her stepbrother, Keiki. She is increasingly attached to her brother, because only he who care about her. Then, she began to seek attention with the people around her, and slowly cultivated exhibitionist behavior.