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Tear Jerker: Tsukiyo No Fromage
Ruri'sbackstory, even if not much of it is revealed yet, it's still enough to draw some tears.
Her first appearance is on chapter 03. Miu saw her walking alone under a heavy rain without an umbrella and with teary eyes, commenting about how her oneesama would punish her if she didn't found something.
On chapter 04 it turns out that the something was Mary, who's a member of a race called Hakubaku (white spirit), who seem to be enemies of Ruri's race the Kurobaku (black spirit). When Miu intervenes and saves Mary from Ruri attack and then tell Ruri to get out of his dream, she starts to sob immediately. Even so Ruri tries to attack Mary one last time but they she's faster and escapes along with Miu. After this, Ruri starts to cry saying that her oneesama will punish her again for being incompetent.
Come chapter 6 and Ruri is still walking alone under a bone chilling rain. But, this time she's shivering in cold and with her face completely red from a high fever, plus her eyes show sign of her being even more depressed than before.
And then, on chapter 7, we discover how deep her wounds go after she collapses while trying to attack Mary again and Miu decides to help her. When he takes Ruri to a bath in to warm her up, a scene that otherwise would be gratuitous Fanservice, turns out into complete Squick when we see that Ruri's body is covered in bruises and cuts.
Still on chapter 7, while Miu is feeding Ruri with some milk porridge, she reveals that she had never being treated with kindness by anyone before in her life and swears loyalty to Miu after it.
And finally, on chapter 12 after they were back at Mary's house and after a lot of trouble, Mary asks Miu to choose between her and Ruri, but after Miu tells her about Ruri's body and backstory, even Mary mellows out and allows Ruri to stay with them, despite their Perpetual Poverty and the fact that Mary and Ruri are supposed to be enemies.
In sum, it looks like that Precia Testarossa has a serious contestant to the prize of most abusive parental figure ever in the form of Ruri's sister.